The risk is that you trade JBJ for a middle reliever and then lose Betts to FA before 2021. Ouch.
JBJ isn't going anywhere until there is a determination on what happens with Betts long term.
I honestly believe his value is higher to the Sox than on the trade market. You keep him until he becomes too cost prohibitive (at 700 OPS that won't be a problem) or you have an internal candidate.
Brian Johnson flat out stinks. Wright is perpetually injured or on some reserve list for beating his wife or PED's. Velaz is fine. I wouldn't want him penciled in every 5th day though.
People who could say "they should have traded him in 2016 when his value was high" are not the same as people who say now "trade 2019 JBJ for a middle reliever".