I think there's a subtle but important distinction between _closing_ the old bugs outright, and leaving them open but marking them as "Verify" with a comment like "we're not sure but it may have been fixed with recent work - can anyone confirm if it's still reproducible?"
So no, just closing them because they'll be accidentally fixed isn't the correct argument IMHO. Instead, they should just admit that most stuff will never get fixed and just clutters the list. Even though that hurts.
A bug report is documentation. It is valuable. It's a gift. If written well enough of course.
If you feel open bug reports clutters your bug tracker, first I would suggest you are watching them from the wrong perspective, and second I would suggest you might need to take advantage of some triage / organization tool (better) like labels or projects.
Open issue doesn't mean "planned" or "will look into this".
Saying this as a software developer. Who has also been reporting some number of issues. Reporting takes time.