The internet was a lot slower 15/20 years ago.

I'm not denying that it was accurate 15/20 years ago, I'm questioning whether it's accurate today.

Maybe there's an argument to be made for it in developing countries, but in my experience, SPAs are way, way worse to use than SSR pages on slow devices and unreliable connections. Buttons are unresponsive for a minute as JS is downloaded and parsed and executed, content can't render incrementally, and the UI softlocks waiting for a failed or timed out API request. People don't seem to be making SPAs to benefit slow and unreliable devices and connections.

I'm not arguing with you, I think most SPAs are unjustified, it's just that 15/20 years ago people really wanted to get rid of whole page refresh and all the problems that brings, which is a laudable aim, but has morphed into "why don't we make our own back button" and nonsense like that.

The justification nowadays seems to be (in truth) "if you take away my tools I don't know how to build things so I'm going to run npm init and go from there".