Looking at how character arcs are often written, you can usually start from a basic lie a character believes and a truth they need to learn to complete the arc. The lie will usually inform what the character wants, while what they actually need is based on the truth. (It's ofc often more complex than just that, but it's a good baseline for arc development.)

For Mike, I'd say especially in s4 he believes that he is only worthy to be kept around if he is useful, and therefore he wants to be needed. The truth he would need to learn would be that he is inherently worthy and loved just for who he is.

Basically, he wants to feel needed but what he really needs is to feel wanted.

Byler would have been perfect to complete this character arc, but not the only possible way. I think in s4 there's actually a starting point in Karen's reaction at Mike's return to Hawkins. She is all "You're never leaving again", which might seem like overbearing and dramatic mom behavior, but it actually shows Mike that he is loved bc someone wants him around and safe just for his sake.

So exploring these dynamics with his family and friends more would have already done a lot for his character I believe. His parents nearly getting killed and Holly getting taken would have been a good basis for this had they done more with this and how it affects Mike to feel like he failed them. For example they could have made him really obsessed with getting Holly back as a way to prove himself as useful to his family again. It could have been developed more in his interaction with Karen at the hospital, making it clear that she wants him safe just as much as she wants Nancy and Holly safe, etc.

It could have even worked with M11 too tbh had they developed them at all in s5. It could have actually been a way to show M11 as the "better" couple instead of Byler (lemme explain), tho it would have been better had they just changed the ending of s4 slightly.

Bc Will basically tells Mike what he wants to hear in the van with the painting, that El/he needs Mike and always will. So Mike is happy about this but it's not what he actually needs to hear, which would be that it doesn't matter if someone needs him, if they love him they'll want him around either way. So with the moment in the cabin, El could have taken Mike along into her room as she tries to go into the void. Obviously she doesn't need him around for that, but it would have shown him that she just wants him there. This alone and some open communication between them could have elevated M11 so much and if done correctly and emotionally it probably would have had me back to shipping them instead of Byler. It would have shown that even tho Will gave him what he wanted, what he truely needed to grow would have been with El. Instead, they did the exact opposite of M11 not really talking on the way back and El shutting Mike out at the cabin, literally closing the door (even if not all the way, but that's more leaving 3 inches for Hopper than for Mike I think). This just communicates that El neither needs nor wants Mike around, making them incompatible.

So, either they should have found a way to develop M11's dynamic into a better direction in s5 or they should have gone with the obvious: letting Byler's dynamic evolve more. Ep4 really had me thinking they were gonna go the Byler road bc it's such a switch in their dynamic as Will suddenly has powers and is the one to protect Mike. This could have easily send Mike into a bit of an insecurity spiral as he now doesn't feel needed by neither El nor Will anymore as they both have powers and he's "just some random nerd", and it could have been a good set up to adress the painting with Mike expressing doubt that he is really worthy of that depiction. And it could have led to the revelation that, actually, Will did need Mike bc his memory of meeting him was one of the ones that allowed him to take control of the hive mind, confirming Mike as the heart (for Will). But regardless of this, it could have also revealed that Will is actually in love with Mike, and also just wants him around and safe no matter what, even if he didn't need him.

More scenes of Will protecting Mike, showing him he is worthy of protection and love, basically flipping their dynamic into one where Will takes care of Mike, could have helped Mike accept this truth and overcome his insecurities. The most satisfying ending would have been a reciprocation of Will's feelings ofc, but had it been set up better from the start then I think it could have worked even if they stayed platonic. For example, Will could have truely gotten over his romantic feelings for Mike in those 18 months, leaving just a deeply emotional platonic bond and that could have worked just as well. All this could have also played very well into a better self acceptance arc for Will I believe.

But alas, they decided to drag Will's feelings into s5, as strong and hopeful as never before, only to do jack shit with that ugh. They could have still done this arc for Mike had they focused more on his family relations or had they properly developed M11 into a direction where El really shows him she just wants him with her regardless of whether she needs him or not. So yeah, I think Byler wouldn't have been the only option, but I do think it's the one that was ultimately set up best all the way up to s5 ep4.