But as long as the platform who need to validate that you're an adult don't get your identity, but just the proof, I don't see what the problem is?
> What is the incentive for the citizen to make sure their authentication isn't shared?
What incentives do people today have for keeping their identifications to themselves? Why aren't we all sharing CC numbers? Because we realize some data is "personal" and isn't to be used by others, like our username+passwords or whatever. This isn't exactly a new concept, just look at how it works for anything else that is tied to you.