opposition needs an argument

The argument is that it's fundamentally a bad idea, not that we know a better way to do it. I realize that's a harder argument to make.

Take porn for example. Above board porn sites generally try to comply with the law. They either perform age verification or block jurisdictions that require it. They verify the ages of performers. They comply with copyright law. They enforce rules against unauthorized deepfakes.

Sites exist which are hosted outside the jurisdiction of USA/EU/AU/etc... legal systems which do none of those things. On such sites, there's a good chance of finding CSAM, stolen content, and deepfakes; I will not list any here. Driving teenagers who want to see porn from mainstream sites to poorly moderated ones is the likely outcome of a successful age verification scheme, and an undesirable one.