Firstly, with the simplistic view that there are a finite number of problems - how do you know that you've reached 100%? Isn't the point of getting more people

Firstly, with the simplistic view that there are a finite number of problems - how do you know that you've reached 100%? Isn't the point of getting more people in because you don't know about all the problems? So how can you ever say "I've found 100%"? Next problem is how do you define a universal usability issue? What might cause problems for one person is a feature for the next. I suppose you could argue you've at least identified a concern, but even then you still need a very large n to say with significant certainty you've found most problems. But it's only ever certainty, unless it's the world's simplest interface.