Your arguments seem to be assuming a particularly bad implementation of a traditional backend.
Whenever someone says "this particular architecture is bad!" they're talking about a bad implementation of it.
The point is whether or not you're more likely to succeed at making a good app with an SPA or a multi-page site. For pretty much all brochure-styles websites and many SaaS webapps you're more likely to achieve success (for every common understanding of success) by using a multi-page architecture because they're usually simpler to implement, they work the way browsers expect things to work, and you don't need to implement some hard things yourself. You can make a brilliant SPA website for any purpose, but often people try and fail. Saying "you shouldn't have used an SPA" is shorthand for "You didn't understand or implement an SPA well enough, and now your web thing is failing to serve users as well as it should, and using a multi-page architecture would have avoided the problems your website has now."
Whenever someone says "this particular architecture is bad!" they're talking about a bad implementation of it.
The point is whether or not you're more likely to succeed at making a good app with an SPA or a multi-page site. For pretty much all brochure-styles websites and many SaaS webapps you're more likely to achieve success (for every common understanding of success) by using a multi-page architecture because they're usually simpler to implement, they work the way browsers expect things to work, and you don't need to implement some hard things yourself. You can make a brilliant SPA website for any purpose, but often people try and fail. Saying "you shouldn't have used an SPA" is shorthand for "You didn't understand or implement an SPA well enough, and now your web thing is failing to serve users as well as it should, and using a multi-page architecture would have avoided the problems your website has now."