Server side rendering with light weight JavaScript. The point you make about most cases not needing it is spot on.
Client side rendering is not strictly tied to single page apps. At a previous company we had rails render out a div containing all the data the page needed to mount the react component. No complex api design, no weird permission issues, etc.
It’s all about applying the correct solution to the problem and that isn’t the web if it involves react.
It doesn’t scale to lots of different things regularly.
Where it exists, the web is probably the wrong solution for the problem.