I know they don't have to be anything more than above average players to give Boston an open window, I just think people overrate how certain it is they'll be able to provide that. What's an above average position player, a 3-3.5 WAR guy?
I wanted to dive a bit into this. Below is the median WAR by position in 2022 among players with 350 PAs per Fangraphs to get a rough idea of what the "average" position player looks like. In reality with platoons, injuries, etc. I suspect the number would creep a bit higher if you take the average catcher AND his backup, but just as a single player range (I tried qualified first but like...5 catchers qualified)
Catcher: 1.6-1.7 WAR (Keibert Ruiz and Christian Vazquez)
First Base: 1.3-1.4 WAR (Garrett Cooper and C.J. Cron)
Second Base: 1.8 (Jorge Polanco)
Short Stop: 2.3-2.5 (Bobby Witt Jr. and Amed Rosario)
Third Base: 2.4 (Justin Turner and Max Muncy, funny enough both LA)
Right Field: 1.5 (Austin Hays and Manny Margot)
Center Field: 1.8-1.9 (Myles Straw and Cody Bellinger)
Left Field: 1.4-1.5 (Austin Hays again and Tony Kemp)
Designated Hitter: 1.3 (Gary Sanchez)
If you limit is to purely full time players, very generally, the values tick up. DH went down, LF's dividing line is still Hays but then Yelich who is 2.2. SS is the biggest difference (Kim was 3.7 last year, so a full WAR and change difference). Very generally I'd say add .5-1 WAR to the numbers above if you want to limit to full time guys who stayed healthy enough and played enough to qualify. So we're looking at 2-3 WAR in most positions for a league average kind of guy - so you range of 3-3.5 for above average position player starter seems reasonable to me, though it might be a bit more above than I thought it was.
To your point though, how often does this happen? Let's set an arbitrary bar of 2.5 fWAR as an above average MLB starter. Below are the Top 10 farm systems from pre-season 2017. I've bolded the players who had at least two 2.5+ WAR seasons per Fangraphs. Players in red didn't make it at all to the bigs. Anyone in regular print has made it to the bigs but posted 0 or 1 seasons at that level. I kind of feel like this is a low bar - very generally if you have a Top 100 guy there's a reasonable chance they get a cup of coffee but that turning into even an above average guy is much less common. This is inexact of course - there is a big difference between a Reynaldo Lopez or Hunter Renfroe or Manny Margot or Alex Verdugo who may not have made that 2.5 barrier but were reliable 1.5-2 WAR guys for a time - and a Kevin Maitan or Zack Collins. Similarly, you get a guy like Josh Hader who BARELY made this metric - some would argue he's a lot more valuable than a middling SS given he's a complete lockdown closer and others wouldn't consider an RP as part of this). Pittsburgh is a good example - none of these guys have had two 2.5 WAR plus seasons but all five are valuable players (multiple 1, 2 WAR seasons) on a team that you don't need to spend $5, $10, $15m in FA to fill. So there's a midground here were I think a ton of the non bolded guys do represent legitimate value and can fill a roster spot well that'll let the team spend more freely to fill other holes
1. Atlanta (
Swanson,
Albies, Maitan, Allard, Soroka, Newcomb, and Anderson)
2. New York (
Torres, Frazier, Rutherford,
Judge, Kapreilian, Sheffield)
3. Chicago (A) (
Moncada, Giolito, Kopech, Reynaldo Lopez, C. Fulmer, Z. Collins)
4. San Diego (Margot, Espinoza, Renfroe, Quantrill)
5. Milwaukee (Brinson, C. Ray,
Hader, L. Ortiz, I. Diaz)
6. Los Angeles (N) (
Bellinger,
Yadier Alvarez, Verdugo, Calhoun,
Buehler)
7. Pittsburgh (Glasnow, Meadows, Bell, Keller, Newman)
8. Colorado (Rodgers, J. Hoffman, Riley Pint,
Marquez, Tapia)
9 Cincinnati (Senzel, Garrett, Winker, Stephenson)
10. Tampa Bay (
Adames, Honeywell, De Leon, Bauers)
So yeah, I think your point is pretty fair. Where Bloom will ultimately win or lose on this is identifying the names in the system who can bring back that valuable MLB talent who may not be the next Judge or Swanson or Buehler and converting that while getting the prospects who will represent that kind of value into the fold. But devil's in the details too. The five players like Pittsburgh's rankings may not have those 2.5 WAR+ seasons but all five are good MLB contributors with real value wherever they are now, whereas Cincinnati or Tampa only had one name to write home about. Obviously, prospects below the Top 100 can also deliver value - Tampa had Josh Lowe, Cincy had Luis Castillo and TJ Friedl, More prominently, Atlanta had seen guys on this list and #8 was some dude named Ronald Acuna Jr. (they also had Max Fried, Austin Riley, Kyle Muller, and some other folks deeper in the list).
Personally, I'd hope the Sox system will hopefully produce one or two of those "bolded" type guys - if they get that and a few other solid MLB role players from their current batch and some depth emergences, and convert some talent to pro talent, they're probably going to be in decent shape. Fingers crossed. We should just be prepped for Nick Yorke or Miguel Bleis or Roman Anthony or Kyle Teel to be replacement level or not make it at all or be a 1 WAR type part-timer as well. It could be the 2015 Cubs system (Bryant, Soler, Schwarber, Gleyber, Eloy, Jeimer, Vogelbach, Steele, Cease, with Addison Russell, Carl Edwards Jr., Albert Almora also in the Top 10)...or it could be the 2015 Rangers (#4 system) headed by...Joey Gallo (Alfaro, Mazara, Brinson, Ranaudo). It's so hard to tell.