I hear you there.
I think that will happen, regardless of a 'critical mass', purely due to the instance based nature. E.g. you want to look at retro gaming content, tips, people's builds, etc. That might have been a 100k+ sub (pulling numbers out my arse), with every 3rd posting being "where do I download .roms from plz". Now that might end up being 20 5k+ communities where there's less noise, and everyone contributing is more likely to be engaged with.
I do think that presents potential issues with sustaining users, as people are so used to everything being in one place / end up feeling FOMO because they're not subscribed to every single community and don't want to deal with that, but I think it definitely looks good for the actual quality of discussion on any given topic.