Sure it can, but is that the way these companies are using it? Are they using Javascript to improve experience and development speed? If a Twitter textbox is lagging while someone types, then that's a degradation in user experience, and a pretty fundamental one. I like autocomplete too, but I also like my text box not to lag, and maybe there's a middle ground?
I wouldn't say I'm a "purist" about this stuff, but I do feel like at least scripting should make the site feel better, not worse. A lot of these sites are really unpleasant to use, Youtube swapped over to this single-page model that I'm sure was very difficult to build, but sometimes it just breaks navigation for me. What fixes it is I reload the page. So it feels like, to me, even as someone who has Javascript enabled on Youtube, we have gone from an experience where navigation never broke for me on Youtube, to a situation where sometimes Youtube loses track of the fact I'm online and I need to refresh the page, or I hit the back button and it reloads the same video. That's a lot of extra Javascript for an experience that feels like a step backwards. The old Youtube designs all used Javascript, but they also had working back buttons and felt just generally snappier.
I went on Google maps recently and tried to quickly copy a number of website links for businesses into a text file: right-click copy link doesn't work. I can't copy a link for an external website, Google Maps is no longer using hrefs for literally the primary thing they were designed for.
And for what? It's not faster for me to open the external websites, they don't load quicker. I don't get URL previews or some crud. It's a link that doesn't work as well, all that's happened is there's a click handler that has fewer features than the link used to have, and maybe it's easier for Google to get a pingback? This isn't an improvement.