Happ needs to be the starting LF next year with his 35 HR potential and decent fielding skills. Schwarber needs to be traded to clear up the logjam. If Schwarber had played fulltime this year he woould have been on pace to have the 5th most KOs in the history of baseball
Happ probably doesn't have 35 HR potential. He's more a 25-30 HR guy best case and the scouting prior to this year had him far lower than that. Not 100% why as he'd shown 20ish HR power in the minors. As for Schwarber vs him, people always want to make it about one young guy vs another but that's not really how a roster works. You frankly want a lot of overlap in players both in terms of injury coverage and in terms of depth for days you rest guys. Next year I think what you're going to see is Happ take most of Jay's role opposite Almora in CF. Almora is going to play vs LHP and probably some more vs RHP but he's not going to play 160 games or whatever. He's at 319 PAs this year and I think given his recent performance you could argue for him getting 400-500 PAs next year. Jay has 427 PAs this year so if you roughly split his playing time with Happ that's 200 PAs Happ can easily get.
As for the rest of Happ's PAs, think you can fairly reliably play him as a rotational piece at 2B. Baez is going to be Bryant's back up at 3B so when Bryant or Baez sits Happ can play 2B. And chances are you're going to sit Schwarber vs some if not all LHP so that opens options there.
Long story short if there's a guy left out in the cold to me it's not Schwarber it's Zobrist. Zobrist is going to be 37 and is hitting .238/.323/.384. Schwarber obviously has some flaws but is hitting .208/.313/.462 on the season and .241/.333/.553 since the start of june. You give Schwarber a healthy offseason where he's not rehabbing a knee and I think you will see those numbers since the start of june improve even more. If he cuts down his k rate to say 25% he's probably something like .270/.360/.590. And the thing is Schwarber wasn't a high strikeout guy in the minors. That's important to note because typically speaking players eventually return in line with their minor league numbers after they fully adjust to the majors. Mentioned this before but right now his swing doesn't handle anything on the inner third of the plate. But that's the same exact problem Rizzo had prior to 2014 as a lefty slugger. Schwarber has work to do there but it's not like it's impossible to fix that.