If you reject the "it's not that bad" response then you are saying all bugs are equal in severity / urgency / priority.
Which is obviously not true.
A better response might be something like "I understand the problem is X however the fix might take Y months and the next release will deprecate that feature anyway. Can you quantify or guesstimate the impact on our product if we allow this bug to stay unfixed?"
Yeah, in retrospect, I disagree with myself, too. (and agre with your rationale!) :)
I think if I were to try and salvage the quote, I'd change it to "It's probably not that bad". In this telling, I'm meaning to imply that the responder has made an apriori value judgement about a bug without determining scope and cause.
I agree with your ideal response. The only caveat I'd personally make, is that I'd recommend "planting the flag" by providing your own guesstimate of product impact, and ask the reporter for their opinion on moving it.
In my perspective, it's important to acknowledge that is has an impact, and it's especially important to convey your sense of impact concretely with some napkin math. If you're terribly wrong, the reporter will try to convince you of it[1], and you'll learn something along the way. It also conveys empathy (i.e. we're both trying to solve your problem, even if it's only a problem for you).
Which is obviously not true.
A better response might be something like "I understand the problem is X however the fix might take Y months and the next release will deprecate that feature anyway. Can you quantify or guesstimate the impact on our product if we allow this bug to stay unfixed?"