This was a huge and, honestly, disgusting retcon thrown at us in the finale. Everything about Mike and El's relationship showed us that the biggest problem in their relationship is Mike seeing her as some mythical figure, a superhero, instead of who she really was or could have been. Right up to the end, he clung to a fantasy of them running off in a manner similar to a D&D campaign.
But he somehow understood her better than anyone.
Better than Hopper, who acted like a father to her and genuinely seemed to see her as a daughter, not just a replacement for Sarah.
Better than Max, her best friend who encouraged her to figure out who she was instead of trying to fit the roles that people wanted her to fill.
Better than Will, her brother, who was the only one who could really understand what El went through, and vice versa.
No, somehow it was Mike, who put her on a pedestal so high that he felt even he was unworthy of her, who had no idea how to talk to or even be a friend or boyfriend to her when she wasn't in superhero mode. Somehow this extremely unhealthy relationship was genuinely meant to be seen as a tragic one that would have succeeded if not for circumstances beyond their control.
It's so very convenient, isn't it? By canonically making all of their problems external, they can gloss over the fact that they were always an absolute train wreck together.
And this isn't just me being some anti who is pissed about Byler not happening. While I am mad about that, and likely always will be, this is about them trying to force something that has no narrative foundation.
Everything about Mike and El's relationship screamed that they'd never work together and would eventually grow to resent each other. They followed the same path as Ted and Karen, who had a hot and heavy relationship devoid of emotional connection that relied on expectation and fantasy. They went as far as to portray them in situations similar to Ted and Karen, such as the dinner scene in season 4.
Mike never understood El. He only understood what he wanted her to be (El was much the same with him), and he came to realize that it didn't match the reality in season 4. However, they never do anything to show that either one of them ever grew with each other after reuniting.
So, having them act ambiguously broken up and platonic throughout season 5, going as far as to give them entirely separate stories that keep them apart, only to throw that void scene and an "I love you, Mike" at the end and just leaving it at that is a completely disingenuous copout. It's left just as unresolved as so many other plot lines in the final season.
Mike ends up mourning a girl who he never treated right who the creators saw as more of a concept than an actual person. That's uncomfortably similar to how Mike saw her, to be fair.
But what Mike should be mourning is his childhood being trapped in such a terribly written story.
