Comment by btasker

Personally, I think this is the answer too - rather than mandating it across all platforms, they could have created a service which provides scanning so that there was an additional app people could choose to install (and would, presumably, present as an accessibility addon so it could access content in other apps).

That's not without its own issues though - creating external deps is more or less what they did the first time they tried to mandate age verification.

Although their plans fell through, they created an industry who'd expected a captive market and started lobbying heavily. Eventually, it worked and we've ended up with mandatory age verification.