Well I mostly agree this should be the first steps in building an app.

But there are a lot of areas where interaction with JS shines over form posts.

For example a form where a choice determines the next steps in the form.

This could be done with simple form posts but then you loose your scroll position or at least it will jump because anchors are not at the same position as your scroll position.

I believe JS is overused and most of the time Single Page applications could be Multiple Page applications with JS where it is needed.