I have been in similar situations and agree 100% with you on the technical side. But they may not have been actively engaging in self-harm. If a dev isn't very experienced in front-end, it's easy these days to fall into the trap of creating an SPA without realizing what you're doing, just because that's what libraries expect you to do by default. For example, any library that supports client-side routing without discussing the alternative.

You might have been able to turn things around for the company with your experienced view.