Users don't have to use a self-owned addresses (planets). They can use moons which are delegated by the planets that issued them. The only limitation is that they are not fully independent (can't be resold, can be "canceled" by their planet) - just like an account on the website (just being run independently).
> not to mention the 8-year-old daughters or 12-year-old daughters or 16-year-old daughters who need conditions on their digital freedom.
Nothing is really stopping them from using a moon issued by their school, university, a random website offering such an account, other family member or a friend etc.
Right now most of the world is using identities issues by Facebook, Twitter, Github etc. and not many people seems to mind. Also most people have their devices behind a NAT and also don't mind. That's exactly what using a moon would mean.
Having a planet is for more serious usage: where one wants to guarantee independence: proper identity that can't be canceled and so on. Akin to owning a root DNS name for your personal server. So 4 billion is probably even way more than we are going to need, even if world population grows x10.