This is going to be a short and sweet little history of Reddit. Reddit was founded in 2005.
Take a look at what Reddit looked like in 2006: https://web.archive.org/web/20061206235353/http://reddit.com/
Note that it didn’t have subreddits back then because the user base was too small.
Look at Reddit in 2008 (December 31): https://web.archive.org/web/20081231080128/http://www.reddit.com/reddits/
Politics had just 72,314 subscribers. Technology had 85,678 subscribers, and the “Nicher” Food subreddit had only 4,438 subscribers.
Lemmy/Kbin follows the same path. Initially, generalist communities like Politics and Technology will have the most momentum and gain subscribers, just like Reddit did back then. As the user base grows, “niche” communities will be able to sustain themselves.
Let’s not think about the Reddit of today, let’s think about Reddit of old. Rome wasn’t built in a day.
Users will only subscribe to a couple of those though, so It’ll be a competition amongst them all. Collections with the best content, discussions, and moderation will win.