Why is the fandom so sure that Renly would have doomed the 7 Kingdoms?
The elephant in the room are the Others and the Long Night they bring. Unless it's a damp squib like in the show, that's an apocalyptic event that will likely reshape society and the political landscape considerably, but that's not on Renly. If anything, Renly was Westeros' best hope for the Long Night because his victory was all but guaranteed to be quick and decisive if not for the shadow baby, thus leaving Westeros united to face the Others, for what's that worth.
However, even ignoring that, Renly married Margaery Tyrell, thereby sealing a direct alliance with the Reach and several of its most important houses. That, paired with a politically smart marriage for the firstborn leaves very few people who can actually plot a "repeat" of Renly's succession. That powerblock would bring a lot of stability to the crown (assuming Renly doesn't repeat Robert's mistake and keeps Storms End as royal domain).
I've seen a lot of talk about dangerous precedent, but the matter of the fact is that the Seven Kingdoms were built on the principle of "might makes right" when Aegon forged the realm with dragon fire. Robert became King with his warhammer. And Renly rebelled despite a lack of a "younger brother" precedent, so it's not like Renly invented the concept. Maegor arguably pulled that stunt, but even he didn't invent the idea.
And there are a lot of historical precedent for states that had a succession hiccup which didn't matter in the long run. I feel like a lot of fans are grasping at condemnation because Renly dared to usurp Stannis' claim, and they like Stannis.