The great thing about SPAs is that your server now just returns a simple JSON API. It greatly simplifies the server side. It also tends to make things a lot easier to unit test on the server side. Also, it cleanly segments your staffing requirements. If your app is in some way non-trivial, you can have domain experts working on the backend, and just dumping json into http responses, and have a frontend engineer who doesn't understand any of the magic work on the frontend.

> Returning HTML and doing a simple element replace with the new content is 99.9% indistinguishable from a SPA.

Once you're doing this, your backend engineers are dealing with html in addition to whatever their real job is. If you want to have something other than your website consume your backend, you're rewriting stuff to output json anyway.

If you have no domain-specific computation happening and your service will only ever be consumed as a more or less static website, serverside web frameworks can be faster. For example, if you are building a blogging site, or maybe a CRM.