4chan did not incubate redpill. Reddit was the primary spot for redpill to congregate, until Reddit started cracking down on TRP and related subs. Cherry on top, Reddit has not acted against the highly similar FDS and many other subs.
Incel culture is also skeptical at best. Yes, 4chan did have the famous Elliot case, and several boards were famous among them. However, Reddit also banned several subreddits related to them which made them flock towards 4chan.
This doesn't change the issue (rampant media exposure is bad for the human psyche). It just puts into perspective things have been going downhill since the masses got access to social media and unlike 4chan, the darker corners of Reddit, Twitter, Facebook and whatever else aren't nearly as taboo and may even sneak onto the homepage feed randomly. 4chan was mostly the place where the extremists would congregate, but extreme cases don't explain why the number of moderate or higher cases would increase.
To put things further in perspective: 4chan started in 2005. Twitter founded in 2006. Reddit, 2007. Smartphones took off somewhere late 2000s/early 2010s. The "average" kid did not have access to these platforms 24/7 before 2010. Those that would likely didn't spend the little PC time just browsing the darkest corners of the net on a single monitor.