"I'm not convinced the problem is too many topics - The main problem is that everyone sticks stuff in AIBU"
Exactly - that's why I think MNHQ should do a little more moving to the topics.
There are a handful of topics which were created specifically so they could be hidden. They can be very quiet between times, so they are falling foul of this kind of 'snapshot of numbers' cull (thinking primarily of The Royal Family topic - which was acclaimed by all sides because it was easily hidden by those who didn't like the deluge of threads, but all were easily found for those which did). MNHQ have however refuse to continue the previous policy of moving threads there. And now want to scrap it.
So I think there is perhaps,a wish to have just a handful of juggernaut topics.
The suggestion that a 'megathread' would replace a topic is also, to me at least, a strong indication that this is not being driven by someone with a real a real in derstanding of MN as a community with disctinct posting habits. Instead, it's just a numbers game.
The small, quiet, niche, valuable support topics are currently being gunned for too.