I think that "I'm a backend person" says it all -- most of the people in this thread are probably not JS natives or front-end natives. All of the complaints about JS fatigue, or not knowing how to use client-side routing are because you've done things differently on the backend. Is it such a surprise that front-end frameworks and tools make more sense to people who actually enjoy and have experience working on the frontend?

If you provide an API schema, with a SPA the whole frontend can be developed separately of ever touching backend code. Okay, so you changed the name of model.thing to model.anotherThing - quick change in the reducer that is consuming that end point and call it a day.

You have to deal with multiple languages in any case: Java/C#/etc for the server logic, HTML for rendering and CSS for styling. You also have to deal with whatever syntax you are using to embed expressions in the template. If you use SPA there is one additional language (unless you use Nodejs for server logic) that's true. And you also have two different build chains. But the advantage is that the server logic becomes much simpler because you don't need session state or rendering logic. Your server can focus on being a stateless REST service which is more scalable (since you don't have session state clients can connect to a different server on each request). Your server is also more future-proof because it won't need to be rewritten to support native mobile clients.