I disagree that very few users would want it sorted that way. Many people
I've talked to who use Gmail cite the way they handle email threads as the
biggest reason they use Gmail. If I get a reply from someone to an old
thread, the thread is now by definition the most recently active thread in my
inbox, so it should be sorted as such. I don't want to miss new emails I get
simply because they were replies to very old messages. I would argue that
this is in fact a usability bug.