A few things on the Braves strategy front.
I think there's value in separating out those Braves extensions. The Albies and Acuna extensions are criminally bad for the players. Ozzie Albies got a 7/35 extension with two club options on the back end for his 22-30 years and Ronald Acuna got an team 8/100 extension with those same two club options for his 21-30 seasons. Both Acuna and Albies had 4ish bWAR seasons under their belt at the time. I think any GM in the game makes these signings. This isn't just team strategy - if Rafael Devers' agent had gone to Chaim Bloom in the offseason in 2019 and said Rafael Devers would sign for 8/120 with two club options on the back end Chaim Bloom would probably move anyone on the team to make room for that. Honestly, I bet if Triston Casas would take 7/35 with two team options on the back end they'd do that right now even with minimal MLB track record. Those two are truly absurd deals that I don't think we'll see again any time soon.
The Strider, Harris, Riley, and Olson deals are probably a bit more in the realistic realm but I think also highlight how much of this is player specific.
Austin Riley is an interesting one with Rafael Devers on the mind - another young third baseman who had a pretty meteoric rise. He signed a 10/212 extension in August 2022. Riley's bWAR by year
It's similar, at least through the first three years, to Devers who had .9, .2, 5.4 respectively. However Devers fourth year, where Riley signed his extension, was a down year (COVID also). .7 WAR. This is where Bloom would have to make a decision like the Braves did with Riley in terms of service time, but Riley was in the midst of another 6 WAR year at the time. Giving Boston the ability to see 2021, it gets iffier because now Devers is two years away from FA now and Devers, kind of like X, seems likely to bet on himself in this spot as well. I would posit that if Devers was wlling to take that Riley deal, he'd be a Red Sox into the 2030s even in 2021. Obviously now given how the market changed around. But my point here is that the Braves locked up Riley on a deal with a 21m AAV who is putting up 6 WAR in two consecutive years - I think Austin Riley probably would've commanded a LOT more than that on the FA market this year for instance. It's a below market extension
Matt Olson is interesting because this seems to be what Boston wanted to pay Raffy seeing him as a 1B/OF convert in the future. He was acquired from Oakland and then given an 8/168 deal with a similar club option on the back end for his 28-36 years (two arb years I think). Matt Olson also cost them four prospects, the biggest of which were Christian Pache (#12 on MLB's 2021 rankings) and Shea Langliers (#73 on MLB's 2021 rankings). One wrinkle to consider the Braves were not willing to meet market on their 32-year old decade long hometown superstar (Dodgers signing for 6/160ish). But yeah, it's a weird mix because it is a very good deal but the acquistion price is a wrinkle since their unwillingness to meet the hometown talent cost means they had to deal, and possibly lose the ability to hold long term, two fairly valuable prospects to replace some of Freeman's value. Dansby Swanson isn't a perfect comparison but let's say he wasn't an FA and the Sox could lock him into this kind of deal...but it'd cost Mayer and Yorke to do it? A bit less appealing.
Michael Harris is more like the Acuna and Albies contracts but there's some interesting new wrinkles - some of them highlighting the benefits of this but also possibly signifying players considering it a bit more. First, Harris' extension costs more immediately - not much for his value but his base salary is 5m in his two min salary years and 8-9 in his arbitration years. Great deal for Atlanta but shows an adjustment for Harris as they got Acuna on 1m for those years. Harris also has salary escalators on the club option years to where, if he remains a 5 WAR OF (or even settles at like...3 WAR) it's still a deal.
Spencer Strider is entirely different. It was a 6/75 extension which is overpaying his arbitration years in exchange for an extra couple years on the back end. It may prove to be savvy but this one is a lot riskier for Atlanta in my view and might represent more what teams would encounter normally - a player willing to lock in a large sum of guaranteed money for a year or two discounted on the back end, not to the level of Albies, Acuna, or even Harris.
It's also pretty important to note that all of these players had all-star level seasons in the book or in nearly complete progress prior to getting these deals. Of course, so did Devers, Betts, and X so it's not a matter of difference on that but getting those 4, 5, 6 WAR seasons in rookie years or year twos isn't terribly common in itself. - Devers and X were both in Year 3 closer to Riley than Acuna or Harris and arbitration years are staring them in the face.
This isn't to make excuses, I think the if Bello or Casas are 3, 4, 5 WAR players next year and the Sox think they are viable and the players are willing to take team-friendly deals with team options in the back or buying out a couple years of FAs? Absolutely. We should also remember these smart teams are willing and able to let talented, hometown players walk who won't agree to these deals. Like X is heartbreaking, but my understanding is Freddie was a whole 'nother level in Atlanta. This is part of the equation sadly. Atlanta's FO is very smart but Atlanta's FO also seems willing to let players go at FA who aren't willing to severely compromise their value or who bet on themselves in the FA market. Maybe the Sox need to be closer to the RIley market than the Olson market on Devers...but I kind of feel like that Devers would've either been locked up on a pauper's deal in 2019 or 2021 (relative to actual value) or hitting FA in 2023 in Atlanta.
Edit: To the point above, the last Red Sox second year player to put up a 4+ WAR was Mookie Betts in 2015, the last first year player to do it was Daisuke Matsuzaka in 2007 which is a very different scenario. The last true minor league rookie/FYP to do it was arguably Nomar in 1997 if you don't count 1996 which was a partial season. It's rare. Your players putting up those numbers in years two and particularly three is more common but you then have the Devers issue where some players will lock it in and some won't.