Amazon still runs a bunch of Perl in the back. Just because you're stuck with some design choice doesn't mean that framework has a solid future. People still write plenty of COBOL but you don't see any new applications using it
Plenty of new mainframe code is written in corporate settings. Legacy modernization is strong as ever and for some companies that doesn’t mean moving off mainframe - it means upgrading, cleaning up and cloning legacy COBOL code bases and some times writing new.
I'm convinced RPC frameworks are the future of REST. It's too crufty without them. Most of the rails stack just isn't involved in those calls, so there's not much purpose using it.
We use gRPC web, so backend is a bunch of RPC endpoints. Front end is regular SPA.
This has worked great for us because it reduces backend to "shipping objects" instead of all the bs you normally have to deal with in HTTP.
With gRPC there's no use for rails, or any HTTP server framework. As far as I can tell, this is the future of web endpoints, so rails will die out.