Certainly, I'd tell him we love him, appreciated his input to the team's past success, and will want him back in 2021, but if he holds a grudge, so be it.
1) I think we get way more than just one decent prospect by trading him. Yes, the team getting him pays about $28M, but he's worth way more than that for 1 year in the heart of prime, and the one year is less risky injury-wise.
2) Getting just a 4th round draft pick by watching him bolt via free agency would be a kick in the balls.
3) Offer him a great deal after 2020, and if he refuses and signs for ungodly numbers elsewhere, I think Sox fans will not be that upset- not like the Lester fiasco.
4) If we don't get Betts after 2020, it's not like we'll pocket the $30M+ for 8-12 years. We will spend it elsewhere, and I like Bloom's history of spending money wisely. That $30+M alone is more than he had per year with TB to spend on FAs.
The worst case scenario is getting nothing for Betts and watching him bolt.
The second worst might be keeping him and resigning him to $36M x 10-12 years and going bust.
Better is to trade him for 2-3 prospects or young ML ready players that are cost controlled and then make a more than fair offer in 2021 and let the chips fall where they may.