> how do you respond if they ask for proof that nobody has fallen off the edge of the earth?
The same way you should respond to all questions. Truthfully. You have no proof, and you don't care to waste your time looking for it. If that is a showstopper for the conversation, it was never worth having in the first place. Simple as that.
I think that's practically what will happen a lot of the time, but it's not very satisfying. It seems to amount to "this person won't be convinced by my unconvincing argument, so the conversation wasnt worth having". (Unconvincing in that you're basically asking them to take it on faith rather than demonstrating it rigorously).
The same way you should respond to all questions. Truthfully. You have no proof, and you don't care to waste your time looking for it. If that is a showstopper for the conversation, it was never worth having in the first place. Simple as that.