Just a guess, but I think they will try and thread the needle with Miggy’s playing time next year. Not release him (mostly for PR) , hope he rests up and can have enough health to perform at a low, but not awful, level. Limit his games, with Carpenter and/or Meadows able to DH. See if that keeps the knee healthy enough to produce close to what he did for a short stretch earlier this year. But be ready to DL him (60 day?) if he falters to the level he’s been on for the last couple moths.
There are roster composition problems with carrying him that way. Maybe he gets 200 ABs and sits a lot. Maybe he objects and it all blows up. I just don’t see them cutting him loose to start the season, or going forward with the idea he’s the regular DH. How big is the eye of the needle?