@jon abbey - thank you for the note on bWAR and fWAR - truly appreciate the help on trying to make new posters into better posters.
@EvilEmpire - I'm the one missing something, not you. You're absolutely correct.
I don't want to take the words out of someone else mouth, but the comps I see listed (just isolating out the most notable and recent players) of similar batters through the age 25 seasons include Eric Chavez, Ryan Zimmerman, David Wright, Evan Longoria, Troy Glaus, and Nolan Arenado.
Chavez stopped being a dependable, useful player after his age 28 season. This is the "worst case" of the bunch. Zimmerman started breaking down around age 29, save a bounce back year in 2017. Wright started missing some real time in his age 28 season, but did remain a mostly productive full time player (two seasons of between 100-115 games) up through his age 31 season. Rolen missed a ton of time his age 27 and 30 seasons but remained a productive player through his age 35 season. Longoria broke down after his age 33 season - and he was about average his last 3 full time seasons. Glaus missed significant time his age 26 and 27 seasons, but otherwise was a reliable middle of the order bat through his age 31 season. Arenado just complete his age 31 season which has been the best of his career so far. Probably Rolen or Arenado are the most "optimistic" outcomes.
The thing we don't know is how much any of those players might have been (or appeared) more durable with the DH. Zimmerman (Washington), Wright (Mets), Rolen (Phillies and Cardinals), Longoria (Giants, for his age 32 and 33 seasons). I believe Arenado had 16 starts at DH this past year.
I guess if one thinks Devers is "more like" Chavez, Zimmerman or Wright, you should want to deal him for prospects. If one thinks he's more like Longoria or Glaus it's up in the air. If one thinks he's more like Rolen or Arenado, you should probably want to extend him at whatever it costs. But I'll admittedly leave that "projection" up to the plenty of people on here smarter than I am. Looking at those similarities and knowing that Devers has the option of moving to 1b and eventually DH - the latter of which some of the others did not - I'd lean toward extending him.
I will say the one thing I DON'T want to see happen is to enter the season with Devers on the Red Sox and in a walk year. Either decide he is part of the next core and extend him or deal him for young players whom MIGHT become part of the core. Don't lose him for the relative nothing of a comp pick.