Reddit doesn't care if the niches leave, they don't drive $ anyway
Most of the casual users I know use Reddit for a few small meme subreddits and topics that are too small for their main-platforms. If those are down, Reddit is dead to them.
(Of course Reddit could try to send their staff each down a different rabbit hole to fit them into the communities. This would be fucking hilarious. I hope this doesn't happen but if it happens, I'll have my popcorn ready.)