If the cap doesn't rise significantly, the question is really can you afford to keep him? Talent-wise, he's a keeper.
Contract-wise, can you afford to keep paying Matthews $12M, Tavares and Marner $11M, Reilly and Nylander $7M, and Muzzin and Brody $5-6M and still have money to ice a competitive team when Campbell will ask for a shit-ton more cash after this season, you need to re-tool your backup goalie situation (Mrazek M-NTC @ $4Mper), Miheykey is a UFA this year, and you've got four other RFA's in Kase, Engvall, Sandin and Liljegren to bump up? Bunting and Kampf also turning out to be keepers with only one more year on their contracts to becoming UFAs.
For a while it looked like Willy Styles might be odd man out, but his play this season has changed a lot of minds (not mine, don't like him) and his contract is too sweet to deal. You're not going to trade Matthews or Tavares (NMC) or Reilly (NMC), Muzzin (NTC), Brodie (NTC) either.
You can keep playing the shell game with the bottom six, but in the end something will need to give. Thinking of the future I still say trading Marner this off-season for a boat load is probably the best option.