Welcome to the internet, the same thing could be said about every web forum since the 90s.

> There's not much to do aside from start a rival subreddit

Yes, that's the system working as intended. You don't have any right to the subreddit any more than the people who got there first. There have been many successful offshoot subreddits that go on to eclipse their predecessors in size if the new subreddit is actually better.