Yeah, no, it doesn't take a genius to figure out that they don't actually give a shit about whether you're "old enough" or not. They just want to have your ID so that they can even more decisively link your data with your person.
While I can't speak for everyone, I'm pretty sure a lot of people simply won't be watching age-gated videos any more. I will never give the cronies at Alphabet more information than I already have, and if that means missing out on watching a few videos, then so be it – either creators start uploading those video to another, proper platform, or they're likely to be missing out on a lot of views.
In the end it doesn't matter if YouTube's the biggest video platform or not, because if you don't have anyone watching your videos, then you might as well settle for a lesser platform where users actually watches what you upload.
Update #1: The latest master branch of youtube-dl seems to have found a solution to the new age-gate. So, if you want to watch a particular video that's been age-gated, then just fire up that and download the video. If you're one of those GUI fags, then you can try tartube, which not only acts as a GUI to it, but also so much more.
Update #2: youtube-dl is dead, long live yt-dlp! Run: pip install --upgrade git+https://github.com/yt-dlp/yt-dlp.git@master and you'll find the age-gate fixed again, for now.