This would protect children while only minimally infringing on privacy.
The mechanism by which we make everyone 'just' is laws. The laws that are being passed are telling of the actual goals.
However this makes the layer even more load bearing for de facto access to having a regular life in society, so I wish to see a legal framework for privacy/security as well as appeals process for the painful edge cases where people get locked out.
In fact its worse. Every site must also implement this security check. Or everyone must agree to just use sites and services that follow this policy. Otherwise anyone can just use another, often 'less safe' website.
Your example confuses the locus of control. The platform is making the choice and relies on user inaction rather than action. Users as a whole basically always descend gradients, and if they like / are addicted to the service, they'll descend with enough momentum to carry them over one-time friction like an ID check. The null hypothesis is they continue using the service. For it to be an "if everyone just" answer, it would be "if everyone just decided to stop using these extremely sticky services" because that is the de facto choice they are presented with. And it similarly suffers from an "if everyone just" lack of plausible mechanism.
The point of calling out non-solutions masquerading as solutions is to keep people's energy focused on possible but unstated solutions, rather than spending time blaming people for behavior largely determined by myriad immovable circumstances.