Mike's hidden arc that he doesn't need to put away "childish things": D&D, games, and his relationship with Will
During the promo build-up for s4, Finn Wolfhard said that Mike was struggling to be "as normal as possible and keep on a normal path" during this season. Which can only really be read as his relationship with El: the other thing he pursues in s4, D&D with the Hellfire Club, is NOT something that makes him fit in with the crowd!
Throughout the series, we see a tension between Mike and his conservative, emotionally-distant parents who do not understand him and insist he "grow up." They scold him for getting into trouble, they take away his Atari, and make him get rid of his children's toys. Mike doesn't want to let go, however:
And in s3, who represents holding on to "childish things"? His best friend, Will.
Mike gets wrapped up in his relationship with El and ignores D&D. He tells Will, "We're not kids anymore. I mean what did you think, really? That we we're never gonna get girlfriends? That we're gonna sit in my basement all day and play games for the rest of our lives?"
And Will says, "Yeah, I guess I did."
To which Mike has instant regret. And we find out by the end of s3 that he still wants to play D&D after all - Mike didn't fully believe what he was saying. It reflected his parents' pressure on him to "grow up."
And of course, in s4, he's in the Hellfire Club and he's all about D&D again.
What changed? El, who Mike got preoccupied with in s3, is far away in California. He was with El for only a year, and much of that time was apart. For the rest of his 15 years, he was a Mega Nerd, and now he ends up with "the freaks" in Hawkins High. This arguably is the "true Mike Wheeler."
And we see what Mike's struggle to be "normal" in s4 is. He wears a garish outfit he normally would never wear, that Argyle calls "a knockoff." He's awkward with every idea El proposes: burritos in the morning, and of course, roller skating!
Many viewers in the General Audience have complained that Mike's character takes a nosedive starting in s3. This is when he enters the relationship with El, which is also when we see less of who he is - "the heart" of the Party who kept the Party together. What makes him a "hero" is something that society might look down upon in a young man: his AFFECTION for his friends, especially Will. That side of him recedes into the background when he is wrapped up in his relationship with El.
But by the end of s4, Will reminds him of who he is. He tells Mike that he's "the heart." And Will says HIS dream (while illuminated by the sun), something that Mike deep down wants, too:
For Mike, Will represents happier times and his own lost innocence.
Because of Will, by the last episode Mike is going back to his hero self: he's helping El while piggybacking, and grabbing Will's shoulder and making clear they'll kill Vecna together.
There's a very famous quote from the Bible: "when I became a man, I put away childish things." And there's an even more famous response from C.S. Lewis:
Mike is struggling to find out who he is, which includes what (and who) he loved as a child.
Will's painting is the final clue as to where Mike will end up by the end of the series: being the "heart" of the party again (Finn has confirmed there will be "leader Mike" moments), playing D&D, and probably being with the person who loves playing D&D with him and loves him for everything he is: Will.
This is a show that says to hell with society's expectations. From the beginning, it has contrasted the bullies and abusive dads with our heroes: the nerds, the "freaks," and the outcasts.
Lucas' whole arc in s4 was going back on his decision to try to fit in with the popular crowd. Mike's arc is to embrace the fact that he doesn't have to abandon who he is to "grow up." He doesn't have to worry about what society wants him to be. He can love his nerdy hobbies; he can fight for his friends. And he can dare to love the boy (a BOY!) -- the same one he, many years ago, with the clarity of child, proposed to in kindergarten.
