Quick note on 2 - the rumor was they would have taken AVT at 16 had he been there or someone else and taken Davis Mills in round 2 had they been unable to select Mac at 15. AVT was scooped up right before they picked. We'll never know but I think Fields was too much of an issue for them as a pocket passer and needed too much development for them to want him. I was a big Fields guy. I underestimated the amount of work he needed//thought he would develop quicker.
I know this is long but I think the Sony question is an interesting thought exercise.
We will probably never agree on 1. That's how hypotheticals works though: I can say with a lot of confidence that Sony was the product of the offensive line and blocking and wasn't that spectacular as an RB. But that's only 1 part of the hypothetical - how replaceable he is. The other part is how good the rest of the team was and how important his individual contribution was. In your argument he's both irreplaceable and his contribution is so significant that they can't win without him.
We have 2 main grades for offensive line play from Football Outsiders. Football Outsiders has adjusted line yards and open field yards:
Unlike other FO stats this one is not a black box:
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Open field yards:
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How you interpret these together:
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The Patriots had the 3rd highest adjusted line yards and 25th highest open field yards indicating they had a very good offensive line and bad running backs:
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Individually he was 24th/47 in DYAR for RBs with over 100 carries and 26th/47 for DVOA. That's not horrible by any measure but it doesn't indicate he was anything other than an average back and again RB DVOA is also tied to the offensive line which we know is very good.
Let's look at pro football focus:
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4th best in run blocking by grade. So both film graders and the analytics agree the Patriots had a top 4 run blocking OL.
I am not crazy about PFF's individual player grades but of the 17 RBs with over 200 rushing attempts he graded out at 11th/17. Of those 17 he was 10th in yards/attempt with 4.5. He was tied for 15th/17 for yards after contact with 2.65. He was 13th on the list for breakaway percent (% of carries that go for 15+ yards), at 24.1%. I've defined elusiveness elsewhere but he was 14th in elusiveness too for the season with a 30.4 rating. He was useless as a receiving option only having 8 total receptions for the year. The median backs had 50 and 43. He was always good in pass pro - I don't want to discredit that. Excellent even in pass pro. That was his best trait...
So when you look at him vs other bell-cowish backs he didn't excel in playing through contact, breaking off big runs, or being elusive. Meanwhile he clearly played behind an elite run blocking line. And while being a check-down option isn't a be-all and end-all he was a detriment to the team as a check-down option and receiving option and it's possible a replacement player would have given them more production there even if it is just a marginal upgrade.
Ok... but what about the playoffs? Let me say that none of the RBs graded out very well in the playoffs which makes some sense because you're talking about the best teams and some of the best defenses. We're also talking about a tiny 3 game sample size. Losing teams will often not have impressive running stats. So these come with a grain of salt.
Of the 15 backs with 10 or more carries in the playoffs that year he was graded 5th highest. His Y/A of 4.7 were 3rd best. His yards after contact improved from 2.65 to 2.90, 4th best. His elusiveness went up from a 30.4 to a 32.2, 7th best. His breakaway went from 24.1% to 25.3%, 6th best. None of these rates are impressive compared to the regular season rankings but he at least was better.
The Patriots OL graded out as the best run blocking team in the post-season at 78.4 which greatly helped.
How about the quality of opponent defense? PFF doesn't adjust anything vs opponent defense. FO does. The Chargers were 28th against the run by DVOA, the Chiefs 18th, and the Rams 3rd. PFF had the Rams run defense 15th, Chiefs 30th, and Chargers 26th. We get some conflicting data here although it would appear the Chiefs are somewhere near average to below average, the Chargers were bad, and the Rams were average to very good (huge range). PFF can't adjust vs opponents so it's possible the Rams, for example, played a really tough slate of opponents with good run games. Altogether though the Chiefs and the Chargers weren't good run defenses and most likely below averageish for the Chiefs and bad for the Chargers.
3 of Sony's TDs came against the Chargers and 4/5 Patriots TDs came on run plays on a day they passed for close to 350 yards. If you're telling me they needed Sony to win that game I'm really skeptical given they were winning 38-7 at one point and their offensive line completely dominated the Chargers in run blocking and pass pro. Vs the Chiefs he had 2 TD and over 100 yards but only 3.9 yards per carry and a long run of 11 yards. Much less effective. They also ran for 4/5 TDs on a day Brady once again threw for nearly 350 yards. I'm skeptical they couldn't have thrown for those TDs or a different RB couldn't have accomplished what he did. Now vs the Rams he had 18 runs for 94 yards, 5.2 yards a carry, a long of 26 and a TD. To me this was his best game vs a solid to very good opponent.
So how important was his contribution and how replaceable was he?
Running backs are one of the more dependent positions in the NFL. Running backs are greatly dependent on their lines minus a handful of truly special talents like Nick Chubb or Rhamondre Stevenson. Sony Michel was not that guy. It's hard for me to see how a below average to mediocre running back who had 2 good games in the playoffs was so crucial for them that they don't win the SB without him. Maybe they beat the Chargers 35-28 instead? The Chargers were a shit run defense. Maybe another RB produces well vs the Rams? He didn't do anything that impressive vs the Chiefs. So saying they don't win without 1/22 starters seems rich to me especially given we're talking about one of the least important positions on the field and most replaceable. The Patriots defense held the Rams to 3 points and the Chargers to 7 before they gave up a bunch in garbage time. And again he averaged 3.9 yards a carry vs the Chiefs which isn't anything special. He had a good playoffs. I won't take that away from him. But a lot of guys had a remarkable playoffs. It is unlikely to me that they couldn't have replaced him and his production with a different running back, a running back by committee or passing more vs the Chiefs.
Sony, an average to below average RB who had 1 to maybe 2 good games in the playoffs and was not a factor at all as a passing back, was a small but meaningful part of their championship run at a position that is the easiest or one of the easiest to replace. The offensive line, the passing game, and the defense also had good games at points and individual players on those units also had some spectacular games.
While it is true you can't say "anyone could have replaced him" you can say it is more likely than not that an average quality back could have replaced his production behind their excellent offensive line. They took him, he performed, and they won the super bowl is unfortunately, to me anyway, bordering on post hoc ergo propter hoc and a logical fallacy. They signed and drafted a lot of other guys who contributed to their SB run. Is everyone now irreplaceable? If your answer is yes then we just will have to agree to disagree on how we look at things. There is nothing championship caliber about picking Sony Michel. He was a below average running back who had, again, 1-2 good games in the playoffs. He never made a pro bowl. He was replaced by Damien Harris. He was shipped out of town for 2 day 3 picks. You don't know and I don't know if they win or don't win without him - I won't be able to prove that nor will you be able to be able to prove they couldn't have won without picking him. Maybe the Browns take Sony Michel instead of Chubb and Chubb falls to them? Maybe they trade for a veteran. Maybe they find a UDFA, give him the opportunity, and he turns into an average guy. It's impossible to know what they would have done or what could have happened. I can only point to his pedestrian stats, other factors like a dominant offensive line, and conclude that I think you're probably overvaluing his contribution and that he was likely replaceable even though he had an above average playoffs (I think - it's hard to untangle how good the line was vs Sony but let's give him the benefit of the doubt). It's also unclear how much he actually helped them win vs all other 21 starters. Personally I think the OL, defense, Edelman, and Gronk were more valuable. And that TB12 guy. If you argued they don't win the SB without Gronk, TB12, Edelman, etc, I have no qualms with it. Those are very good players and unlike Sony it would have been extremely difficult to replace their production in a hypothetical. They also play much more valuable positions that are less dependent on other factors like an OL. WRs are somewhat QB dependent but good/great receivers still produce with them (See TO, A-Rob as two examples). But you're telling me they don't win it without a mediocre RB and that's just infinitely harder for me to buy.
There is an element to hypotheticals that is probably frustrating which is it's a lot of speculating on unknowns. I'm comfortable with this particular one but it's ultimately a thought exercise and YMMV. It's unlikely that you or I are going to change each others minds but I at least want you to know where I am coming from.