That’s called "moving the goal posts". A definition of memory safety that permits unsoundness as long as nobody has exploited said unsoundness is not a definit

That’s called "moving the goal posts". A definition of memory safety that permits unsoundness as long as nobody has exploited said unsoundness is not a definition that anyone serious about security is going to accept. Unsoundness is unsoundness, undefined behavior is undefined behavior. The conservative stance is that once execution hits UB, anything can happen.