All the big adult sites will probably just die or at least shrivel in popularity. Most Europeans simply will not use whatever "tell Brussels or London where or what what you are watching" option is. In the place of the big sites there will be a billion shady and likely virus-lottery proxy sites whose only selling point is that they do not do age checking or require registration. Those then get occasionally smacked down by Brussels, just to be replaced with 10 more clones the by the next week. On the side piracy and vpns will thrive. Kids will not be protected nor will people's privacy, quality will be worse.

I would also bet that when the landscape decentralizes there will be a lot more cp, revenge and peep-videos and other illegal shit in the mix that will get through through the cracks since massive established sites had to actually fear shutdown and losing all revenue unless they had robust gatekeeping mechanisms. If Brussels wants your 2 month life-expectancy site dead anyway, because of it's only selling point of having to show id, then why really bother with the quality control of the material. Especially if site holder has no personal qualms about that stuff.