Any idea that is based on "If everyone just..." is wishful thinking. Describe the mechanism by which you convince everyone to just do something.

Pass a law that requires devices and software to support a per-device or per user account 'child' or 'minor' flag. The flag must be lockable with a password or another account. Pass a law that mandates that websites and content handle the flag appropriately, whether that means denying service or limiting access.

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.

Sure, but the ID solution is an "if everyone just gives up their privacy / anonymity / sensitive data" and the mechanism is by denial of service

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.

I'm not advocating for that either, I'm only pointing out that "if everyone just" is a collective action problem that is a non-solution because it doesn't describe the mechanism by which everyone does something.

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.