I actually still don't really think that's a problem, even knowing it took up that high a percentage of Recent Posts. It just sounds like raging against the dying of the light. I know people freak out about Recent Posts & Recent Changes getting too much material in them, but that's always gonna happen. They're not gonna stay useful for long as our site grows.
I'd rather unlock it, but in this case, it's no big deal. If some other popular thread gets locked because people think it's clogging up recent posts/recent changes, though… that isn't going to be awesome. You can't grow a community by trying to force down activity level. Silliness like "Thread" is part of that.
A possible alternative: Institute some minor rule or rules against proliferation of content-free posts. (Ex: Posts must be longer than one sentence, etc)