I wonder if people understand just how much of tumblr's userbase actually came from 4chan.
Think about it. It's 2011-12, the Great Cartoon Horse Purge just drove a bunch of people off /co/, /v/ is in the middle of the console wars between the Xbone and PS4, /b/ is shit like always but is losing appeal with young people, people in /r9k/ who actually want to get their shit together recognize the toxicity of that place and decide to leave.
Enter this new shit website that people keep complain about. Sure, it's filled with kids which might actually be the appeal for some of 4chan's userbase
and crawling with the "essjaydubs" which only served to bring it more publicity
, but it's also got tons of porn, artists that migrated from DA and livejournal, a bunch of MLP roleplay blogs
, and was quickly becoming the new watering hole for all the new fandoms that were popping up. And on top of it all it was one of them newfangled Social Media doohickies but doesn't require any real life information like Facebook or Twitter, it even had an app for your phone! Wowzers!
So all the young people who would have come here five years earlier started posting on tumblr instead, and many older 4chan posters who grew tired of the site's negativity, who stopped finding racism funny when puberty ended, and gays because /lgbt/ didn't exist yet. These people decided to leave in an effort to get off the ride, little did they know they just brought the ride with them. And the problem was, the only people who were left were the ones who didn't have a problem with those things, and thus /pol/ and /r9k/ became the new dominant boards on 4chan.
Fast forward eight years and now tumblr is the left-wing equivalent to 2011 4chan. I can guarantee you if the site had lived for a bit longer it would have gotten its own Hackers on Steroids moment.