I think you're 180° backwards on that.

How many alternative platforms are there really going to be that can afford to develope and operate the legally-mandated age-detection ML-models?

Especially after the bureaucrats see that the law isn't working and start looking for scapegoats without massive lawyer teams to make an example of

Why would they do that? There are plenty of platforms that simply won't care, and there's stuff like Mastodon et al.