I think the biggest reason to move him off of CF is that he misses a lot of time, not that he's merely adequate out there now. It would be the same story with a SS, or to a lesser extent 3B (Catcher also but that's a whole different category here)
Those are the hardest positions to fill on the fly. When he inevitably misses time, you're scrambling to field an adequate defensive replacement. Him missing time becomes a double-whammy of missing his bat in the lineup and then either putting an empty bat in the lineup that's capable of playing CF or moving one of your reasonable bats to CF and crossing your fingers defense.
That's not the case if he's your primary LF. Sure the defense probably takes a hit when he's out. but the list of guys that can play an adequate LF is way longer than the list of guys that can play an adequate CF. Finding a replacement for 1 game, 1 week, or 1 month shouldn't be very difficult.
I fear two things though:
1) They will defer to Buxton, who will not want to move from CF. I get it. He's earned that right a little. He's a 10+ year vet who's the face of the franchise. It'll be a tough conversation with him. Have him be a great defender at a non-premium defensive position rather than an adequate defender at a premium defensive position. Frame it that way. Let us ooh and ahh for the next 4 years over what an awesome defensive LF he is.
2) The Twins will be wishy-washy with it. Gotta rip the band-aid off. Don't make it part time. Once he's moved from CF, make it permanent. Can he roam around out there in a pinch? Yes. The same way that Lewis can still technically man 3B and Keaschall can technically man 2B. But don't make it part of the plan. Move him and be done with it.