Earlier quoted context omitted.
Offline use already works great, unlike in any SPA I've ever used. 1) Load the page. 2) The page is now fully loaded and ready for reading without any need for further network requests. For even more offline use, try printing it! HN threads work superbly when printed, again unlike most modern sites.
I don't think printing is a viable solution for most users. I'm guessing most would want to download the comment threads to their phone so they could read them later in situations where connectivity is patchy (eg train/plane etc).
Or you could reimplement newsgroups (but with less interoperability) as an SPA, which in theory would work great (once you work out the kinks) but in practice would suffer from all kinds of little issues in the from-scratch reimplementation of everything. For example, printing would probably forever be a wishlist item in the backlog.