>I think everyone can agree that having O(100M) people vote on every local initiative is absurd.

I'm one of those everyones, and I don't agree.

Except if you mean local initiatives that don't concern 100M people, but e.g. some regional municipality. Of course then just the locals can vote, be they 100K or 1M.

Yes! I meant local issues that don't concern 100M people. Local issues that concern a few thousand people can be (and often are) resolved by direct democracy.

I guess I could argue that putting a stop sign at a particular intersection in rural Kansas could concern me, even though I don't live in Kansas, but I think very few people would make that argument in good faith.