Mike's future is so sad, but the worst part is that he knows it.
He tells his friends little epilogues for their characters that are obviously thinly veiled ideas for what he thinks they deserve. Dustin goes off to college and gains the knowledge he craves, but also makes time for adventures. Max and Lucas settle down and enjoy being with each other. Will finds his place in a big city where there are others like him.
Maybe the others noticed what Mike was doing or maybe they were too busy enjoying the visualization of their endings. Will, though. Will noticed. So he asked what Mike's ending would be.
And it's just sad. He just keeps writing stories, except for the one he can't.
Mike isn't happy in his ending. All he can do is come up with happy endings for others. Mike's ultimate fate is to end up passively going through life, never believing he can have any for himself.
He probably ends up just like his father.
His stories for his friends could really also be things he wishes he could have: personal fulfillment, love, belonging. These are things he saw his parents never really had, after all. He's giving up and accepting his fate, just as his parents did before him.
Hell, the endings for his friends even have them away from him, as if their finding happiness requires that he not be around. Even the ending he imagines for El has her far away in a fantasy he imagined for her.
Mike is basically saying goodbye to his friends there. He's wishing them well, letting them know how great he believes their lives will be. Meanwhile, he'll spend night after night, day after day, sitting at his typewriter, giving countless people the happy life he doesn't believe he can have.
It's hard to believe the Duffers think El had to leave for Mike to grow up and move on. The future they wrote for him is exceptionally bleak.
I suppose the only ones who can save him are the fanfiction writers.