I need to be honest: I really hope Byler is endgame because Byler makes me hate Mike less.
This is a long rant, and this whole post is gonna sound full of vitriol, but BEAR WITH ME, I HAVE A POINT.
I hate Mike's character so much, the show does him so dirty, S3 was THE WORST and S4 was only better because of the Byler implications, because otherwise he still sucks
Allow me to rant after the cut before you guys jump onto me, ok?
He used to be the leader of the party, and in S3 he does NOTHING, and seasons 3 and 4 seem to go out of their way to show DUSTIN (my pookie, this kid is the GOAT, no one say NUTHIN' bout him) as the leader instead of Mike, which like, makes Mike kinda disappear into Mileven? He goes from leader of the party to El's only safe haven, and that bothers me so freaking much. It's a disservice to both their characters IMO. El gets stuck being the only one who can do anything to save the day, and Mike gets stuck being the only one who can be there for her. I don't think it is a coincidence that they get separated for most of S4, because I think if they were together the whole time, the plot would stop moving. Mileven in canon feels like being in a relationship just because you don't know what you would do if both of you weren't together, not because you actually like each other.
Now if Byler IS endgame, everything feels more cohesive. Mike is an asshole, yes, but not because that's who he is - he is an asshole because things are changing and he is hurting his best friend and he is really bothered by that but he doesn't know how to fix it. He avoids saying stuff not because he doesn't want to, but because it feels wrong and he doesn't understand how. He didn't want to play D&D with Will in S3, and he said it was because he was maturing, but as soon as he enters high school, he finds a D&D club and is super dedicated towards it. Lucas has his big game and Mike doesn't even think to negotiate with Eddie to move the last session around so they could all watch the game AND have the finale of the campaign - clearly D&D is, still, a huge priority for him, despite him being technically older and more mature now. Yes, he is still a dick for replacing Lucas at the last session, but this assholery puts his previous assholery ("Did you think we would be in my basement playing games the rest of our lives?") into a new perspective. He STILL cares about D&D, it is STILL super important for him. So why was he doing all that in S3? Makes you question, and that's good! Also very interesting how this priority only comes back once El is not around anymore. It's like he is back to his little bitchy S2 self, maybe trying to get back to the way he was back then.
S3 Mike is overprotective of El because, according to him, he "loves her and doesn't want to lose her". But, if my interpretation of Canon!Mileven is correct, he might be actually scared of what not being with her means for him. If Dustin is more of a party leader than he is, if he and Will are not on good terms, and he doesn't play D&D anymore, that means he's not Leader Of The Party Mike, nor Mike The DM, nor Will's Best Friend Mike. That leaves him as Protector of El Mike. So, he's overprotective, because that's who he's supposed to be, now, right? That's what having a girlfriend means, right? Lucas and Max are always together, and they are dating. So Mike should be there with her and for her, always. If El breaks up with him, who is he then?
So he fights to get her back. And since he's fighting to get her back, that probably means he loves her, right? When someone breaks up with you, and you still want them, no, NEED them back, that means you love them, right?
But then she leaves, and he can'tdo anything about it. And Will leaves, too. And suddenly, he's not missing the time he spent with El. Something feels wrong, and he's still a little bitch to everyone around him, but it's not because of EL. He talks to El, he writes her letters, they keep in touch. And he doesn't miss her. He should, because she's his girlfriend, and he's supposed to be missing her, so he complains about not being able to call her on the phone, but he doesn't try to solve that problem like he did in S2 when he didn't even know for sure if El was alive. S2 Mike would move the Earth to be able to talk to El. S4 Mike thinks just getting on a plane to see her in the summer is enough. A letter every once in a while instead of a phone call is enough. "From Mike." is enough. S4 Mike didn't implode while not having El, like he thought he would, so things change. He's not as protective anymore because he doesn't need to be. The last year was enough proof that he could still be who he is without El, so he has another angle to self-reflect.
If Byler is endgame, then S4 Mike being an asshole to El after she defended herself in the rink is less of him just being a shitty boyfriend and is instead him being mad at himself for neglecting his friends and his own self to take care of her when she truly doesn't need him, even without her powers - and then taking it out on El. He understands that he is pissed, he is prompted to make amends (iirc he didn't go talk to El out of his own volition, he just did it after he saw how Will was reacting to how bad he was treating El) and he can't. But he CAN make amends to the one he thinks he actually wronged: Will.
I hope we get to see a healthier (platonic) relationship between Mike and El in S5. I'd love to see Mike stop being the little asshole that he is after understanding himself a little better. It would put a nice bow in things because GOD MIKE YOU ARE SO ANNOYING AND SUCH A JERK RIGHT NOW.