I’d argue the disaster was caused by front-end developers. “Let’s replace a perfectly functional website with megabytes of JavaScript browsers now have to parse & execute on every single page load.”
Technically the framework and payload is gonna be cached by your browser. That they don't server side render anything means all that JS has to load all the content into a big engine, which takes up a bunch of network calls. Also tracking + ads.
The framework & payload still needs to be initially loaded which can be a problem on unreliable mobile connections. Unlike a classic HTML page where a single request failing just means an image or font is missing, in this case the entire SPA won’t load and you’d be looking at a perpetual blank page.
Server-side rendering still means the browser has to parse & execute a ton of JS after the initial SSR-rendered page is loaded which is still going to be slow unless you’re running a big CPU with lots of RAM.