Let‘s say you have 5-7 dependencies to load, but 3 of them timed out because your train entered the tunnel. Your app ends up in incorrect state, fails silently and UX degrades unpredictably. This is where the conversion often drops visibly and the reason SSR is now a go-to solution for any marketing website.
And if they're deliberately going somewhwre where there's no signal for an extended period of time, and really want it to work, they'll ensure they've loaded everything before doing so.
And I say this as someone who is developing a pwa that is for people with low end phones and very inconsistent and/or connections. I'm very cognizant and empathetic to their situation.
Regardless, this isn't really restricted to the usage of JavaScript. The website would likely have pretty bad UX if only half of the CSS loaded correctly, but no one programs defensively around it being absent.
An user opening a web page is not expecting a full-blown app with multi-second loading times. If that happens, they bounce, and you loose revenue. Web is supposed to have very short time to first content paint and very short time to interactive, the shorter, the better, less than 0.5s is the goal. It can deliver that, if built properly. Many SPAs, bulky JS apps are built this way for developer convenience, not for end users. The only real use case for SPA is when you deal with a lot of local data. A spreadsheet, document or image editor, a diagram tool (but then wasm is probably a better choice).
You may say, you are not building enterprise grade frontend. But if you are small enough, you don’t need SPA either.
Downvotes should give you a hint that the few users that know what javascript is, don't like it, and the rest of them, if they learned, most likely won't like it either. Your attitude shows that you don't care.
Let me get this right, you're saying that people on HackerNews don't know about JavaScript-- one of the most popular programming languages in the world?