That's exactly my argument...? Seems to be a misunderstanding.
> I'm not sure why pretending there isn't can somehow make a system less complex.
Yes, that is exactly why pre-SPA web programming was more complex.
> Furthermore, how do you write automated tests for your modals and slide-out panels? Your test suite needs to essentially run a browser. This is absolutely more complex than having an application on the server compute the document that the client expects.
How do you test that the HTML/JS generated by the server actually show a modal? At least with libraries like React you can test everything up to the actual CSS engine. And no, you don't need a browser for that.
> How do you test that the HTML/JS generated by the server actually show a modal?
You can test for the existence of markup on page load. You can't test that some JavaScript for displaying a modal has run without running a JavaScript runtime, i.e., a [headless] browser.
In most cases, just loading a page which displays the appropriate information for the user is cheaper to test and implement, and usually is a better experience for the user also. It depends of course — this would not be true of an application like Google Maps. Most web applications aren't Google Maps though.
…But there is a network in between. I'm not sure why pretending there isn't can somehow make a system less complex.
Furthermore, how do you write automated tests for your modals and slide-out panels? Your test suite needs to essentially run a browser. This is absolutely more complex than having an application on the server compute the document that the client expects.