The last time I saw this come up (that I recall) was here. The suggestion then was to become a pmdevil and write the patch yourself. I think the suggestion remains valid here, too.

In the thread above, I followed that advice, and wrote the patch. The code aspect for that was easier: all the information was already stored in the node data. This request is a bit more than that - you would need to scan the database for each node you're looking at. I don't know the exact structure of the database, but if "parentnode" isn't indexed (which it probably is), this could be quite expensive. Someone who is a bit more familiar with the underlying database would have to tell you that.

Then, of course, is after you figure out what to do, is to convince The Powers That Be to actually apply your patch. If you're going to affect the performance of the system, I'll bet they say no. Especially as you can elect to get messages for each reply, and delete those messages as you are finished with them.

If it does not affect performance at all, they may decide that they're getting something for (almost) nothing, and apply it.

Here endeth the advice.

Personally, I'm ambivalent about it. Mostly because I get those response messages. It would be nice, however, if I got emails about responses to responses (i.e., a thread I'm originator to is active). Well, sometimes it would be nice. Other times it would be annoying. :-)