The BIGGEST and I mean the most prevalent argument in any place where even people who consider themselves to not be GA bring up for why Byler is unreasonable is that Mike doesn't reciprocate Will's feelings and actually loves El. They say that Bylers are scrapping for subtext when Mike doesn't even have to be gay for anyone to sympathize with his story, a nerd with bad attachment issues, coming up in a time of conformity where he has to be the ally that his friends need when all authority figures would rather dissuade him. Mike has show no signs of being interested in Will in that way and if Bylers can't see that what Mike and Will have is a very strong friendship built up for childhood, with a lot of history and Mike is just naturally expressive and emotionally forward, then they don't see it. Mike might have a lot of emotional intimacy with Will but that is just from their strong friendship and emotional comfortability that comes with years of knowing each other and is only platonic, it doesn't have to be more than that and there have been no signs that it is more than that. Mike has shown romantic feelings for El, even if they lack the emotional intimacy in later seasons.
1. "The BIGGEST and I mean the most prevalent argument in any place where even people who consider themselves to not be GA bring up for why Byler is unreasonable is that Mike doesn't reciprocate Will's feelings and actually loves El."
This is true. The GA and mileven shippers often disregard the possibility of Byler because "Mike doesn't love Will, he loves El," which, surface level, makes sense.
But Stranger Things, again, isn't real life, it's a story, and in a story, everything happens for a reason.
If Mileven was meant to be the most lovable, relatable ship, why did they make the writing choices that they made in season 4? Let me explain:
If Mike was really supposed to overcome his inability to say I Love You to El in a way that represented a healthy relationship, the writers wouldn't have made it so that, instead of dealing with the situation by confiding in El herself, or someone like his mom or even Lucas(who's in a very good relationship), Mike confides in Will, his best friend who's in love with him.
It's hilarious when people dismiss Byler even though this very fact is right in their face. Why make us root for Will? Why make Mike confide in Will, knowing that Will is in love with him?
2. "Mike doesn't even have to be gay for anyone to sympathize with his story, a nerd with bad attachment issues, coming up in a time of conformity where he has to be the ally that his friends need when all authority figures would rather dissuade him."
No character needs to be gay to be able to sympathize with them, that's besides the point. Having a gay character is simply pivotal for representation, it's targeted to a specific audience to make them feel seen and heard.
Despite Mike being a nerd, there is nothing genuinely abnormal about him that would cause him to be bullied – he's from a comfortable upper middle class family, has a sister with a good reputation, and has a nice big house. In the 80s, hell, even now, people would look past the nerdiness because of his background.
The attachment issues apply, and make a good amount of sense.
Having "being the ally all his friends need" be one of Mike's primary concerns makes sense, but is also counter-productive. This is a problem that every character has, all of them have to be each other's allies and all of them are facing the same threat. Making something everyone's going through a primary issue for Mike wouldn't do him justice as a main character.
3. "Mike has show(n) no signs of being interested in Will in that way."
A) upon reuniting with Will at the airport, Mike makes no moves to hug him despite clearly missing him (it's canon that he attempted to call and Will wouldn't answer because of Joyce). Instead, he hugs him awkwardly, so awkwardly you can even see El making a confused face in the background. Why?
B) In season 2, Mike called El and tried to reach her every single day. But, despite knowing that she was alive, he made no attempt to look for her, contrary to him looking for Will in season 1 even though he literally saw his body being pulled from the quarry. Mike even stops his calling of El the moment Will is in danger, and instead stands as the only member of the party that stayed with him throughout his possession.
Why does Mike stop caring for El when Will gets involved?
C) The obvious tension-filled moments in season 4: the van scene, the bedroom scene, and more. You cannot watch those and tell me to my face that that's a normal, bro-to-bro convo.
4. "Mike is just naturally expressive and emotionally forward, then they don't see it."
You're right, Mike is emotionally forward and naturally expressive – you could even say that he wears his heart on his sleeve. You can't deny that he acts rather strange around Will, and considering his emotional forwardness and blatant facial expressions that never lie, you start to wonder what he feels for Will.
5. "Mike might have a lot of emotional intimacy with Will but that is just from their strong friendship and emotional comfortability that comes with years of knowing each other and is only platonic, it doesn't have to be more than that and there have been no signs that it is more than that."
Emotional comfortability is one thing, this is another. I'm not dismissing it, it's actually very important to Mike and Will's relationship. In season 2, Will lists of the names of people who he says 'baby' him, Mike isn't listed as one. He also tells Mike the secret about Dart and his feelings of 'going crazy' and even emphasizes that he thinks 'the others won't understand'. He only trusted Mike with this.
Similarly, Mike felt comfortable enough to confide in Will about his relationship with El.
But there's a limit as to what can be played off as platonic.
To put it into perspective, let's replace Mike and Will with Lucas and Dustin: two people whove known each other for a very long time and are seen to have emotional intimacy.
Would Lucas ever do this with Dustin?
It would be weird, right? If Lucas couldn't hug Dustin, if he kept looking at Dustin's mouth. It would be weird if they had a flirty conversation in his room....
But when Mike and Will do it, it's "just platonic...?"
6. "Mike has shown romantic feelings for El, even if they lack the emotional intimacy in later seasons."
So, you admit that the couple that's supposed to be the "flagship" of the show, the couple that everyone praises to be the best, lacks emotional intimacy in later seasons? Does this not....alarm you? Why? Why have they started lacking in emotional intimacy?
It's not even just emotional, it's physical too, there were only around two scenes in the entirety of season 3 and 4 where El and Mike where openly physical with each other. Before which, they'd spent the entire season apart (season 2).
But why? The Duffer Brothers have proved time and time again that they're amazing at writing love (eg: jancy, lumax, jopper....) so why would they fail the main ship of the show?
Because love fades, love is confusing, and love is hard when you're under the pressure of "growing up" and "getting a girlfriend" in a conformist town in the 1980s.