but I think a lot of sites will switch to SPAs in the future, since they offer a better UX
Actually I suspect that a lot of these kinds of sites will primarily migrate to native mobile apps.
A full-stack framework is overkill as a backend for SPAs.
I'm not so sure. HTML rendering is a pretty small part of what a complex framework like Rails does. You still need a database/ORM layer, user management and auth, routing, caching, etc. The same things that make Rails productive for traditional web apps also make it pretty nice for building REST APIs.
If you switch completely to native mobile applications you disappear from the web. That's a lot of people and a lot of incoming links. Mobile apps are less discoverable and they put the install time between "I want to see this" and "I can see this". Sites are trying hard to shave ms from the home page loading time, go figure how well it fits into the picture the minute it takes to decide to download something and get it installed. I think they'll go for a two pronged approach: traditional web site with content plus a mobile app (actually at least two of them). Furthermore, my phone can display an infinite amount of web sites but it can store only a not so large number of apps. And it doesn't have anything as convenient as start typing the name of a site to have the browser display the right url, inner pages included.
Actually I suspect that a lot of these kinds of sites will primarily migrate to native mobile apps.
A full-stack framework is overkill as a backend for SPAs.
I'm not so sure. HTML rendering is a pretty small part of what a complex framework like Rails does. You still need a database/ORM layer, user management and auth, routing, caching, etc. The same things that make Rails productive for traditional web apps also make it pretty nice for building REST APIs.