Take Reddit as an example: Reddit became big after Digg fucked up their mainpage and has replaced most hobby forums and discussions on the internet. A competitor to it would have to implement a lot of compliance stuff like age verification and that would be very expensive, making it harder for a competitor to be bootstrapped. That's why Reddit has an incentive to support such regulation, even if it costs them a lot. Once implemented, competition becomes harder and harder.

Aaah, got you, thanks. I'd incorrectly assumed "this" in your original post meant Wikimedia's challenge, rather than the OSA itself.