I've seen no evidence niche subreddits in general are dying because Reddit's growth in general is so strong and a rising tide lifts all boats. I've seen niche subreddits climb in traffic steadily even as their overall ranking on reddit declines.

I just would have guessed new reddit would have started the decline years ago but the apex hasn't even been reached yet. Still I do think the apex is coming just because of the sheer user hostility of Reddit.