The /mlp/ cancer was much worse than the /pol/ cancer, and the /pol/ cancer was a predictable outcome of increased censorship by Moot after the whole gamergate silliness.

4chan has definitely gotten worse, but /pol/ is just a small part of it. The single worst factor in the last 5 years is (I suspect) the normalization of Reddit and the massive exodus from Reddit to historically "weirder" websites, including 4chan. The existing culture wasn't big enough/viral enough to successfully integrate/indoctrinate the new users.