Even if JBJ hit .800 in 2020, there's no way I feel secure that is the JBJ for 2021 and beyond. There's no "non tender" after 2020. He'll be a FA just like Betts. He won't get a QO like Betts. He's not worth $10M. He's not the only top defensive CF'er available this winter.
Even if we bite the bullet and go for one more shot at a ring, I doubt we go over the max line. I still think it makes sense to not pay JBJ $10+M. We'll still be up against the max line without JBJ and will need money to build up the pen and 1B or 2B. I say just get a cheaper but still great defensive CF'er and build up the pen & 5th starter slot.
Also, we can always sign JBJ as a FA (after we trade him this winter) for 2021, if we lose Betts. Or, sign some other defensive CF'er.
I'm a huge JBJ fan, but he's not the only great defensive CF'er in MLB. He's also getting older and seems to be on the down swing curve.