Question: if every customer of bank X withdrew all their funds at the same time, would the bank survive?

No bank would. (This is fractional reserve banking).

Question: If everyone who bought bitcoin withdrew to their own self custody wallet at the same time, would Bitcoin survive?

Yes. It’s how it was designed to be used.

Now which system is inherently more fragile? Who is the fool? The one with some bitcoin or the one with zero bitcoin?