Terrible connections exist. But I fail to see how adding more scripting will always help with that. Service workers are definitely promising, but documentation for common use cases is still lacking.
Even with a terrible connection, I would still trust a server-side app to load correctly on a refresh more than a SPA.
If anything, the minimal use of Javascript is as inclusive as a web app can be.
That said, even though the way service workers are registered is sort of odd, they're pretty trivial to work with once you're there. As far as resources go, MDN is always pretty solid, and this post (https://developers.google.com/web/fundamentals/primers/servi...) on Google's web dev guides is what I used to familiarize myself with them.
Your app has no control over browser connection management. JavaScript you write is aware of idempotency and retry semantics of your app, so it can manage requests more intelligently than the browser, without disrupting the user experience.
So sure, it won't always help if you do it badly, I can see how it always helps if you do it well.