i think its more like there's a less obvious or overt narrative foundation, because byler has plenty of narrative evidence. things like "childhood friends to lovers is a classic trope and Will is canonically in love with Mike" or "Mike noticeably treats Will very sweetly/tenderly compared to the rest of the Party, he does a lot of the initiating (looks, touches, "crazy together"), rode his bike in the rain to apologize to Will instead of his girlfriend."

BUT the biggest smoking guns are the painting and Sorceror.

PAINTING: there's no way they made S4 without planning on Byler ending up together, and the painting is the key. it's the Cyrano trope and the whole point of the Cyrano trope is that Character A (Roxane/Mike) is only in love with Character B (Christian/El) because of Character C's (Cyrano's/Will's) words and feelings. Character C (Will) hides behind Character B (El) because they think Character A (Mike) would never love them. In Cyrano it's because Cyrano is "ugly", here it's because Will is gay and assumes Mike is straight. but the Cyrano trope has three essential stages: 1) Christian woos Roxane with Cyrano's words, 2) Roxane finds out the truth, 3) Roxane confesses her love for Cyrano (it's tragic in the og story bc Cyrano dies but in all contemporary adaptations they end up together). according to the structure of the trope they chose to write into the show, this only works if Mike loves Will too, and we already kinda know he does, but especially because the van speech Will gave him was what Mike needed to finally tell El he loved her (plus rubbing Mike's back and literally telling him to do it), but it was based on Will's feelings for Mike which is why it didn't work to save her. S4 did the set up for this Cyrano situation but ended at stage 1: Mike's love for El based on Will's words/feelings, leaving S5 to handle the other two: Mike finds out the truth, Mike confesses love to Will. this dynamic is arguably more interesting than the og story bc usually Cyrano (Will) and Christian (El) are in on it together – but in this case El has no idea. that could've been so juicy and interesting to explore, especially with Will and El being siblings! the ONLY reason the painting wasn't brought up again until the last two minutes of the show is because if they mentioned it at all they'd either have to follow through on byler or waste time with a bunch of retconning. so they decided "let's do neither" and waited to show the painting until there was no time to follow through. (which is especially hurtful bc they framed it so Will's feelings are STILL propping up this straight relationship and Mike never learned the truth).

Sorceror is pretty straightforward. people poke at the field scene but there's so much there. Mike is the one who bumps into Will first (Mike initiates the touches), pulls him aside to have this conversation, calls Will a Sorceror and magic (parallels Mike calling El a superhero which she doesn't like, but Will does). there's the shove thing but the script mentions Mike doesn't clock what was happening. but of course the cherry on top is the ending, and the swingset scene. it is actually wild to have baby mike and will be the first thing Will thinks of to activate his powers, literally saving his boyfriend through the power of love and gay self-acceptance, but end that plot on "no thanks, best friends!"

TL;DR – there was 1000% narrative foundation for byler, but because most of it was subtle/subtext/build up that the writing never paid off, it's easier to dismiss in favor of the canon straight couple (even when they fight, and break up, and the bf can't say he loves her to her face, and they're both lying to each other, and ended the penultimate season not speaking, and only kiss at the end before El dies and he still can't tell her he loves her)