What are you using for the backend? A SPA has no value without one. Unless you're using something like Firebase, pretty good chance your backend is an MVC app. (I think you're confusing server-rendered apps with the MVC pattern)
Even if you can stretch the definition to fit, is it still useful to organize your code along those terms instead of what's natural in the new ecosystem? (data source resolvers, type schemas, queries/mutations)
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.
That'll suck. Hope your company doesn't use Github.