I didn't use the phrases "0 competition" or "tiny niche", nor did I say that League of Legends was a "dying game" (I don't even know what it is).
You don't have to though because that is what you are implying. You can always find anecdotal evidence of someone's super niche forum for Ford/Toyota XYZ model surviving or having a stable population. But that is not what is important.
What's important is that it seems to be easier to grow and attract users into reddit or facebook as opposed to an old-style forum for an equal amount of effort. In fact I've seen a few forums that had a lot of work spent on it being absolutely dominated by subreddits with a lot less effort.
This does not just affect a few hyper-competitive niches. When I came back to xenforo.com after several years, the first thing I noticed was how much fewer people there are now and how it took >24 hours to get a reply to most of my posts. It was a stark difference from the past.
You can also confirm here that xenforo has taken a nose dive as well in other metrics that can be found.
And I would tell you to look at all the broken links and forums with 2-3 users here.
Get feedback on your XenForo forum. XenForo forum showcasing and critiques only. You must be a customer to start a thread in this forum. Threads about forums with non-business-appropriate topics will be removed.
The issue here is forums as a whole, not the fact that you can hope to get lucky that your niche is so small that your users have nowhere else to go.