There's definitely trade-offs. I actually prefer a proper SPA to pure HTML when on a slow connection and interacting with a site a lot.
Especially when trying to buy something. I much prefer to have a longer initial load and then have everything just work instead of waiting as I navigate between pages, the multi stage checkout, confirmation etc.
But yeah, if I'm just trying read a single article on a blog, preloading everything is pointless. A hybrid site with the initial page being server side rendered + progressive enhancement afterwards is theoretically optimal in my opinion.
These people definitely have an overly rosy view of the past. The sites that are dog slow shit ux now still would be with server rendered html and jquery spaghetti, just like they were in the past.
Ever submit some giant form to get some random error and then lose the entire state of it? Used to be extremely common, even though it shouldn’t have been.