Meaning of the 1s orbital diagram
You are right: the outer boundary is quite arbitrary. There is no any intrinsic threshold; the probability just gradually decreases lower and lower, but never reaches 0. You may draw a sphere so as to have the electron inside with 90% probability, or 95%, or 99%, or any other value as you see fit.

This, in particular, is the reason why atomic radii are somewhat arbitrary too.