I didn't mean niche to be dismissive. I use it in the biological sense, which is kinda close to use case. All technologies are used in niches, and Nature abhors vacuum, so there's a technology for every use case. However, you can use niche or use case or whatever, and it's true that most technologies slowly abandon the mainstream (or the top 20 in rankings, or reach their plateau in number of users with new users balancing those leaving them) and get stuck to a few use cases. You can probably create reactive web sites and even web services with Cobol, but you're better off doing it with, well, Ruby or Elixir. It's very likely that you can leverage multi-core architectures with C, but you'll most likely use Go or Rust, instead of looking for some library or whatever.
And you are true about the PRs. Most of them are addressed, some of them are waiting for OP action. Still, there are a lot that are simply stale, like this one, three months old github.com/rails/rails/pull/33911. And if you see 700 PRs to be merged, you'll think twice (or more) before submitting your own; many people will simply let it go.
There seem to be ~40 devs active in any one month github.com/rails/rails/pulse/monthly, and the balance of opens/closes and PRs proposed/merged is quite healthy, this month. But still, the balance of open - closed is -40 this month. A t this rate, it might take 17 years to clear the PR backlog. That's not unhealthy per se, but it would be interesting to look at past history, and get the number of PRs and monthly contributors... declining numbers do not mean immediate death, it just means that, for many use cases it was used before (say creation of dynamic websites) lots of programmers have moved on and are using something else (say Vue.js, Elixir, or Sinatra). Contribution sustainability is always a problem for fully-volunteer open source projects, and a bunch of use cases are sometimes not enough to maintain a project, not to mention develop it further.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.