That’s the theory being floated, and for good reason. Finn Wolfhard has a full plate if he wants to stay involved with Stranger Things 3. He will be starring in the upcoming horror film The Turning, playing a strong supporting role in The Goldfinch, reprising his role in IT: Chapter 2, and on top of that producing an album with his band. The other Stranger Things kids have far less commitments. This points to a drastically reduced role for Mike in season 3, and with hints already in the air that a major character will die, it wouldn’t be surprising if it’s Mike.
He’s my favorite character and I’d miss him, but it would make for excellent tragedy in season 3. Some have criticized Mike’s role in season 2 — that he just pined for Eleven, hated on Max, and didn’t do much until the final episode. But that wasn’t poor script writing; it was the whole point, and necessary to make his character believable. In order for Eleven’s sacrifice to be felt, it had to devastate Mike and cause him to stagnate. Wolfhard has said in interviews that he and the other actors referred to his character as “Emo Mike” throughout shooting season 2. “He basically stops growing as a person”, says Wolfhard. He’s always in trouble with his family, and half the time he can’t stand Dustin and Lucas for their insufferable cheer. (The season-2 Mike thinks that since he’s so miserable, everyone else should be.) The only company he enjoys is Will’s, since Will is suffering in his own way. Will gives Mike someone to look after and protect as he did with Eleven in season 1.
Mike’s character was handled perfectly in season 2, and we should be grateful the Duffer Brothers know how to write sequels, and make fans feel all the appropriate aches and pains and losses. A director like Spielberg would never have written an Emo Mike; he would have facsimiled the season-1 Mike in a pointless sequel. And he certainly wouldn’t kill off Mike (or any of the kids) in season 3. If that’s truly what’s in store, I’m looking forward to it, and to Eleven dishing out some bad-ass revenge.
UPDATE (May 14, 2018): Finn has finished recording his album, and he has also done his scenes for both The Turning and The Goldfinch. So it looks like he will have plenty of time for the full season of Stranger Things 3. The shooting schedule runs from April – October and is well under way.