It's not bad is it? Have I missed something?
vBulletin had something like this as default functionality, which is probably why I downloaded it and installed it, but then forgot to actually activate it.
Whilst poking about I just found a modification I'd previously installed but not activated that will show you who has replied to a specific thread, how many times and what their replies were.
1. Click/tap on the number of replies for a thread...
2. ..to see a list of who has replied to that...