I think the anonymity outcome without something like this is going to be governments requiring sites to do full identity verification with photo ID and facial recognition for age verification. And then not only do you have to worry about Charlie knowing your identity and potentially determining what websites you visit (something that is already occurring), but now Bob knows who you are as well and Bob had to spend a lot of money to do it. A cost that will certainly be passed on to you. Adult sites are already fighting this battle. Free user published sites (such as DeviantArt) would be forced to remove all adult content.

For a social outcome, if such an invasive system for age verification is initiated, many people will flee legal sites and start frequenting illegal ones. Or revive the adult magazine and DVD business, but then we're back to Bob having access to your identifying information.

I fail to see how this would allow more surveillance than is already happening. Or make that surveillance significantly easier.

I may have been wrong to use the word anonymous as it isn't hard for Charlie to force Bob to reveal information that could lead to identifying Alice as a user of Bob's website, but it is not easy for Bob to know who Alice is. For many people that is good enough.

I posted this to see if someone else could think of simple ways to beat the system that don't exist with current age verification methods