you know, i’m kinda convinced that the duffers just didn’t know how to write domesticity into mileven. like i saw your tags about how they could love and hate love so much and i feel like it kinda ties into this idea that, out of all the romances in the show, mileven has always been something More. like their love is almost otherworldly with their love at first sight and how tethered they are straight from the beginning. the duffers wrote up this grand love story but either didn’t know how to transition into something more mundane OR didn’t care to in order to maintain that “childhood magic” (even though showing el growing up peacefully would do that more successfully)
The crazy thing is - I think the Duffers DID know how to write domesticity into Mileven. I think in season 3, their cabin hangouts were very sweet and domestic, and we got hints of the normalcy in their relationship at the beginning of season 4 through writing romantic letters sent from across the country (like an LDR and people moving away is actually fairly normal, I was never ever worried about Mike and El keeping in touch given they had been through much worse), spending time together at the roller rink and flirting at the pizza parlor, I genuinely just think they did not give shit about showing the natural progression of their relationship and were more focused on the "tragic" aspect of El and their ending especially in the last season. AKA they were absolutely terrible at balancing it/didn't want to commit to it because they predetermined their fate.
And that's what pissed me off. If the Duffers actually had a birds-eye view of their entire story and focused on Mileven as the central couple, we would've gotten some more true-to-life moments from them, but Mike and El are their favorite dolls to torture.
While I think it was incredibly obvious that Mike and El were dating (unlike the rumors that began at the start of s5), they failed to show their calm, happy comfortability these two had settled down into since s3, even in the midst of all the chaos and were singularly focused on the "three waterfalls" plotline for El's ending (literally all of their s5 interactions revolved around that and nothing else, and that's not to say they were thinking about their future in those last few days but given how close they are they surely would've had other more casual interactions/jokes that even Dustin/Lucas/Mike, Steve/Robin, Mike/Robin had) that the Duffers neglected everything else for obvious reasons this season.
But honestly, I don't think it's specific to Mike and El that the Duffers don't know how to write the later stages of a relationship and run out of things to do and need to create contrived conflict and make them break up. It tracks with the writing of their lore as well, they're great at set up but fail on delivering at progression. (@she-who-the-river-could-not-hold agrees they should stick to movies for this reason)
And for the childhood magic thing: BOY, I want to make an entire post about how it did not work for Stranger Things because their childhood was not magical with the USD trauma, El grew up with them and was already a young adult, the kids who Mike was smiling down at in the basement did not represent "magic" because they were fucking kidnapped, groomed, and their bodies were violated??? I agree with you that El growing up peacefully would do that more successfully - my lovely friend @irenemaiorino made an incredible post about how Mike and El's relationship represents the magic of childhood in the sense that its the piece of magic and tether that you take with you into adulthood (and they should have allowed to progress to the next phase of their life with that fragment of their past)
Like honestly, reflecting on my own childhood, I feel so much more like a child now as an adult than I ever did when I was a kid because although I have responsibilities, I have a much better handle on the mental health issues that wrecked havoc on my mind as a teen/kid because I didn't have the language and techniques to understand and manage my anxiety/depression. Now I do, and I still have the freedom to like eat Scandanavian Swimmers for breakfast if I want (which I did yesterday morning) I can't help but feel like the party will honestly enjoy their adulthood a LOT more and feel more magical during it, especially with all the bullying they faced in Indiana from being nerds. And El as well - she will feel more free as an adult coming to terms with her trauma (and obviously starting a family with Mike at a later date)
IDK if that's me projecting my own feelings about childhood/adulthood on the show, but I just don't think El leaving worked the way they thought it would. The magic of childhood metaphor could've been done in a million other better ways!!