So if we are going to say that the best *reasonable* projection for the next 5 years for Soto is that he averages 6.5 WAR/year, what do we think is reasonable to expect Abreu to average over that same time span? And then how much money ought the Sox spend to bridge the gap between those two projections?
After a certain point, of filling your lineup with (maybe cheap) 2-3 WAR guys, you kind of need to go big and pay a premium for those extra wins. If the best way to pay that premium is by upgrading Abreu to Soto is a fair question. But consider:
C: Wong/???/Teel - You can maybe upgrade here by replacing Wong with a good defensive catcher and hope that spills over into making the pitchers that much better, but this is not a position where we need to spend big. Really, what are the options?
1B: Casas - Cheap and controllable. No obvious upgrade on the free agent market. Maaaaybe if you can trade for Vlad Jr. and extend him, but how likely is that?
2B: Absolute black hole last season, but we have Campbell, and Grissom - one of whom will hopefully be an impact player. You could sign Adames to be a 2B, but he was a 3 WAR player as a SS. He's projected to get around $27M AAV. I don't think signing him to play 2B would be a good use of resources.
SS: Story - Solid defender, could have a decent year with the bat. Mayer waiting in the wings. Again, you could try to upgrade with Adames, but is it wise to bring a guy like Adames in for five or six years when we already have Story signed long term, plus Mayer, plus Campbell - who is also an option?
3B: Devers - You might make some modest gains by moving Devers to DH, and bringing in a guy like Bregman, Arenado or Adames to play 3B. But it's expensive, and not an otherworldly upgrade or anything. And this also presumes you're able to move Yoshida.
LF: Anthony (probably) - Top prospect in baseball, looks ready. And if you're going to move him in a trade (I wouldn't), your still looking at trying to replace him with Soto, or one of Hernandez/Santander... or O'Neill.
CF: Duran - still cheap, cost controlled, MVP level season last year. Good luck upgrading on him, even if he takes a step back.
RF: Abreu - Good defender, decent bat. Doesn't hit LHP. Really good trade value.
DH: Yoshida - Injured, pretty bad contract at this point. Probably can't easily be moved. You could probably upgrade this simply by having Devers and others rotate through while Rafaela/Campbell/Grissom/Refsnyder/Romy get starts around the diamond here and there. Or you could bring in a capable, but cheap RHB. Or Teoscar/Santander.
Basically, I think it boils down to... RF/LF (with Anthony playing one or the other), and DH are the most obvious places to upgrade our lineup. Possibly moving Devers to DH makes it a bit more complicated, but Soto and/or one of Teoscar/Santander are the most obvious fits for this offense. Maybe you end up paying him $48M a year for 4 more wins than Abreu would have given you, but those wins have to come from somewhere. How are you adding 4 wins over Story/Mayer? Or Casas? Or Anthony? Or Wong? Or even Yoshida? Let's say the lineup we currently have, with whatever pitching staff we wind up with, gets us between 88-92 wins. Replacing Abreu with Soto would increase that range to 92-96, and that's huge.