"/pol/ ruined 4chan" is the most buzzword phrase imaginable and it's clear that everyone who says it doesn't understand what led to the decline at all. It wasn't some singular event that ruined the website, it was a long and slow decline over several years.
If you go to 4stats right now you'll see the top ten boards all involve stuff like anime, comics, video games, politics, television, sports. These are all hobbies/entities that are culturally relevant right now, and all of their respective industries have taken a massive decline in the last 10 years. Comics have transformed into overpriced liberal propaganda that doesn't sell and is only kept afloat thanks to the 20 billion capeshit movies that have been made. Sports have attendance problems because the leagues have zero parity and the team owners are greedy boomers who want to bilk money out of people instead of delivering a watchable product. TV and film are full of pedophiles who have to bandwagon every political trend to distract people away from their own devious ways. I guarantee if you wander into any forum that isn't your run-of-the-mill hugbox, you will find at least one thread on how X's ratings are down compared to the beginning of the decade.
/pol/ is the biggest draw to the website now because it's based around something that is both culturally relevant and inherently fascinating. Modern day politics is a never-ending clown show that is infinitely more entertaining than anything else out there right now. Shit like anime and comics are culturally obsolete; they've been doing the same shit for nigh on 20 years now and it's successfully chased away all their fans save for a handful of neckbeard nerds. Politics doesn't have that problem; it's palletable to people of all walks of life. Everyone wants to be part of something that matters in the now. Marvel comics lore from the 1980s doesn't matter in the now. Talks about racism, global hunger and deep state corruption do. That's why more people pay attention to it.
4chan didn't collapse because a bunch of crossboarders kept spamming fag and nigger jokes on every board. It collapsed because it built itself on communities that eventually grew out of whatever pop culture crap they were obsessing over as teenagers.