> at any kind of scale half of them will be already fixed, misunderstandings, bad data in,

Here you're referring to a class of bug reports that's uninteresting for this discussion, because they're invalid (i.e. they don't represent an actual bug). We're talking about valid bugs that have not been fixed.

> or related to an unusual setup

Unusual, but ostensibly supported? Then there exists a bug.

These rules are in place because that first class of 'bugs' is the majority of bug reports when you are at scale.