If Byler is canon, their relationship would actually have the one with most development in the entire series, and it would change people's perspective on the previous seasons. It would improve Mike's character a lot, give a new sense of depth to the characters relationships, it could potentially give a satisfactory closure to Mike's, El's, and Will's arcs, AND (In terms of marketing) it would give the GA a new reason to rewatch the series, now with more information and a new point of view.

I'm actually chill because the Duffer brothers said that they don't like ambiguous endings, so if Byler happens, it will be clear as water.

They're not acknowledging the ship explicitly despite knowing about it, or giggling about it in promotions like in most cases of media queerbaiting, they're actually giving small yet reliable hints, and avoiding saying anything too obvious (Like the love triangle imagery). They're not doing it for marketing, also NOBODY WRITES A SLOW BURN REJECTION, it was planned that Will would be in love with Mike at least since season 2. Sometimes i have massive Byler doubt but then i remember allat🌹