Member Rara Avis
It is a good idea, and just about anything is possible - but not all things are easy. More importantly, not all things are worth the cost involved.
Here's an example of the latter. When you post a new topic or a reply, Athena grabs everything you typed (up to about 3000 characters) and adds it to our search index. The search index is just a huge, 40meg file that she uses during the search routines - because the alternative would force her to read every thread in our not inconsiderable database. Adding to a file is easy. Unix knows exactly where the file ends, so Athena can find it quickly and append that post you just made. But what happens if you later edit your post? Does Athena go back to the search index and make changes?
No, she doesn't. Because changing something in the middle of the file is a lot different than adding data onto the end of it. To change the middle, Athena would have to read the whole file, find your original entry, change it, then write the whole file to disk again. In essence, we're not just replacing your edited post, but rather replacing the whole file. That's a lot of work for Athena, and just not worth it for the occasional edit.
If you wanted to be able to search by number of replies, I would have to add that field to the search index, again to avoid forcing Athena to read every thread in the system. That would mean adding one or two characters times the number of posts in the system - making the file bigger and thereby increasing the work Athena has to do every time she reads that big old file. Okay, that's a cost, but it's a cost we could probably live with. Those one or two bytes, of course, are the digits representing the number of replies.
But what happens when someone else replies to the thread? Oh, oh. Now we have the same problem as when you edit a post. To keep the search index current, Athena would have to update the reply count in the search index - which is now in the middle of the file and requires her to rewrite the entire 40meg file. If you think about it, Athena would be writing that huge index file every single time anyone in the system posted a reply. I think our cost just went right through the roof.