The box office numbers and Wills performance prove your 'comeback' theory. Public reviews (the people that really matter) are saying he and Margot carried the film. It's proof if Will gets in an eagerly anticipated movie and performs well, his fans will flock and begin to look forward to his future projects.
We get so hell bent on the fact he hasn't made music that we almost will his movies to fail. Beside the critics reviews, this is a roaring success at the box office and a personal gain in Wills career as an actor to know he can still be part of a summer block buster.
After Earth sucked, Focus wasn't amazing but it was watchable. Concussion was kept under wraps largely by the NFL popularity and ignorance of the issue it addresses, but Will was fantastic in it (minus accent).
I don't see where the decline of his career is. He's in what will probably be the biggest movie of the summer, Collateral Beauty will probably get Oscar attention and Bright is going straight to Netflix which is probably the largest growing audience movie watching option in the world right now. Couple that with the Bad Boys franchise continuing, which will get attention just coz it's Bad Boys, regardless of if Bay is involved. I'd say his movie career looks pretty healthy.
He's also part of the Suicide Squad franchise now too, so that will have him occupied and in good condition at the box office any time a new movie or spin off is released.