I understand your general point, but these solutions are not niche. They cover almost everything needed in web dev. The niche problems are the ones that arise at scale, which is when a business typically has the resources to solve them. (Hint: you don't have to solve them in Rails).
Large app / company challenges don't apply to new apps. Statements like "I wouldn't start a new app without a great concurrency story." or "I wouldn't start a new app without microservices." etc don't make sense, because new apps have different priorities (i.e. finding the fit, surviving, staying relevant).
I went through a phase of building Elixir/Phoenix apps for 3 years, thinking that I will switch permanently, but ended up coming back to Rails due to higher productivity in that stack. This was in 2015-2018, so maybe Elixir/Phoenix has gotten more ergonomic since. I'm not sure.
