There seems to be a lot of people angling to trade Mookie this off-season because it's better to get *something* in value from him rather than let him walk for nothing. This argument presumes that Red Sox can't, won't, or shouldn't sign Betts long-term, but even leaving that aside, that basis of that argument is that the long-term value of whatever prospects Mookie nets, as well as whatever near-term financial flexibility this wins the team in 2020 and beyond, is greater than the value Betts has to the Sox 2020.
In regards to potential prospects won in a trade, what can we realistically expect to get for one year of Betts at $27-30M? The market for that deal would be limited to a handful of teams that have the cash and that are in GFIN mode in 2020 (or who wants the inside track on signing Betts long-term in 2020). I'm no expert, but I don't think the Red Sox get a haul that meaningfully rebuilds the farm or that nets a high-probability impact talent.
In regards to financial flexibility, they would have more resources to allocate elsewhere on the 2020 roster, but they lose a ton of roster efficiency by replacing a 6-8 WAR player at one position with whatever cheaper solution they find to backfill. In theory, they can make marginal improvements with that money at other positions (SP, RP, maybe 1B) that could offset that lost value, but there's a good chance that spending 27-30M on a bunch of mediocre guys on a handful of positions is less efficient than spending 27-30M on Betts in one position and then going with dirt-cheap options everywhere else. (A more compelling argument might be to reset that LT threshold immediately and regain financial flexibility for 2021 and beyond).
Assuming the Red Sox return much of the same team in 2020 that it had in 2018, and assuming that they can make *some* kind of marginal improvements to RP and SP depth, Mookie could be the difference between a championship-caliber club and one that fails to make the playoffs. I'm not sure how the Sox value that in dollars and cents, but the difference in gross revenue for a team that fizzles out in August vs one that plays meaningful games deep into October has to be comparable to the money the Sox would save if Mookie is traded.
In other words, I'm struggling to see how the expected value of trading Betts this offseason exceed the expected value of keeping him through 2020, even if he walks.