Generally speaking divergence with upstream is a problem and we lost these changes anyway in pkgsrc and every standalone GNU toolchain copy.
Agreed that divergence is a problem, but so is having bugs. It's a question of the right balance in each case.
Finding a creative way to make this at least tunable is challenging with the GCC people.
I think this is the real issue. As I understand it, upstream is making a choice that is inconsistent with longstanding norms and against the consensus of an OS community. So if the best that can happen is that we have to patch, that's how it is. It would be really nice if there were a --default-tmpdir= configure argument, but it sounds like they are unwilling to do that.
Basically, I don't think we can go from "upstream is unwilling to do X" to "impose pain on NetBSD" by making "divergence bad" be the highest-weighted concern. We have to say "what is the minimally painful way to cope".