this is speculation but i do really believe byler was in the plans up through season 4 and they chickened out because netflix needed to use stranger things as a strategic chess piece in their larger plans of world media domination.
i mean, the season 4 california plotline still makes no sense outside of setting up the possibility of byler endgame, though i think they were always operating under the idea that they could set it up but never knew with certainty if they'd be allowed to make it pay off.
but stranger things 4 blew up even more than the first season did in 2016 or any subsequent season, breaking streaming records of all kinds and launching into a full cultural landmark (x). we're talking kate bush resurgence and eddie obsession, merchandise-able everything: hellfire club t-shirts and surfer boy pizzas and immersive experiences and, and, and.
look. we can't really talk about stranger things without talking about the greater context of netflix in the streaming wars circa 2022.
in 2022, netflix was losing subscribers for the first time in a decade (x). netflix begins cracking down on password sharing, offering tiers with ads, desperately trying to keep up their lead in the streaming wars even while competitors like disney+ and hbomax were starting to catch up (x).
in other words, when stranger things 4 came out as a smash hit - it was already a crucial pivot time for netflix. frankly, all they had to leverage was their successful ip. and the more they merchandise everything, the more it grows into a star wars/hp level honest-to-god franchise, the more netflix depends on that revenue stream---the more they absolutely would not risk potentially upsetting the masses with a queer story like byler. or, ultimately, with anything all that daring or unexpected when it comes to ending their biggest flagship show.
then we have the duffers recently saying in an interview that they might return to stranger things "in 20 years if we’re all broke and need some money ... We’ve all talked about it; this is our safety net.” (x)
they sold out. plain and simple. stranger things grew bigger than them and bigger than just the story it wanted to be, into a major cultural and (more notably) FINANCIAL chess piece to be manipulated by the powers that be of netflix. it has the kind of potential for theme parks and spin-offs and reboots and tie-ins for decades to come and they simply couldn't risk that by canonizing byler. they make weak excuses about this being how the show was always going to end, say it's going "full circle" simply by not having let any of 4 seasons of development influence the story's ending from what they did in the very first season, effectively erasing years of character development, particularly for el but also for mike and will.
they pussied out. end of.
tldr; capitalism ruins everything because what else is new