The only real disadvantage for the Sox in resetting after 2020 and then re-signing Betts is the free agent market beyond Mookie right now actually looks fairly weak. This has pluses and minuses.
The big minus is that any team looking to drop some serious coin won;t find anything in the way of alternatives. The flip side to that is, those same teams obviously won't have a $20mill player coming off the books, so we won't know who has available budget, and a lot of them have their own internal star players to worry about, such as the Dodgers. (Sign Mookie or extend Bellinger?) That is, unless Giancarlo Stanton opts out of his Yankee contract, which would be a move his agent will be screaming at him not to do. Much like Price, I don't think Stanton is going to find a comparable offer from the non-Yankee teams out there. I doubt anyone else will top the 7 year / $218mill he will still have left on that deal (which could also turn onto an 8 year / $233mill deal if his option gets exercised). The remianing years for Stanton are the higher paid ones on thta contract.
Now I do expect JD Martinez to be more likely to opt out, a his last 2 years are the lower paid ones on his deal. But he might find staying to be the better move, since his last bout with free agency didn't play out the way it was supposed to in the public story (which might have been nothing but a marketing ploy by Boras).
So that leaves only George Springer as a consolation prize, and likely a considerably cheaper one than Betts as Springer is 3 years older and not nearly as good. Or the Sox and the other suitors will have to wait another year for Kris Bryant...