I think you are missing the forest of the trees, regardless of whether or not the success of rails is circumstantial, the fact remains that Rails is in the lead position because you can build a business on top of it.
It is a solid, battle tested, mature framework that one can use and scale it (up to a certain point) and beyond that, it might get harder but scaling is never easy and I am sure you would agree that Phoenix is not a magic bullet when it comes to scaling.
Again,I don't have to recount the number of services built on rails that thousands of people depend on today: Shopify, Github, Gitlab,..
Us engineers think the only thing that matters is your rendering time.. but it's not , the main thing that execs care about is the bottom line and how fast we can push products to production.
All that being said, I agree that Phoenix is great but let's not kid ourselves, we are here to make cash first and foremost.
To any young devs out there, I strongly encourage you to not pick a framework by its hype but evaluate it based on your system needs / maturity.