True, but in practice, the vast majority of advertising-funded sites use targeted advertising, because they use standard ad networks. There are very few ads networks that don’t do targeted ads.
You're assuming that platforms that tried age verification in order to show people targeted ads would lose enough users to make that business model no longer viable. While I would fervently like to believe that's true, I'm not so sure it is. I think there might well be enough people who will hand over their personal identifying info without a second thought in order to continue to use targeted ad-supported platforms, to keep them in business. I hope I'm wrong.
I don't think it'll come down to people being too principled to participate. I think I'll come down to them being too lazy to bother completing the auth ritual.