Go back in time and stop gamergate and the fappening from happening. That was the final straw. Thats what brought massive traffic of outsiders, normalfags, redditors etc here. You could be more autistic and say around chanology was when things started to turn (Maskfags,Anonomoose) but if you look at the traffic numbers from back then it was nowhere near the increase after GG. The election also saw a mass increase from /pol/
The number of phoneposters also now make up a huge chunk of the posters on this site and we all know that when you post from your phone you are not likely to add well thought out quality posts to a thread because phones are not as easy too fastly type on. Low quality posts mean a worse 4chan.