True, we should respect all the resources of a device, at best. Also true for desktop (Slack's memory usage comes to mind).
A major result of the study is greatly reduced bandwidth (and consequently, shorter parse time) compared to the original TeuxDeux, so I'm working towards respecting these resources, for what it's worth.
To be clear, the study does not care about doing SPAs or not. The results are applicable to server-rendered HTML as well (write a function that enable some behavior on an element, mount by class name, done). I agree that many use cases (e.g. blogs) should mostly be server-rendered and only progressively enhanced with JS for some UX improvements.
But highly interactive apps (drag & drop just being one example) will not have comparable UX without client-side rendering. I do not want a 100-1000ms delay (or an error message) after dropping an item in some list because of a server-roundtrip. This is not good UX.
Also, when filling out a form, I do not want to lose data or context when clicking submit while I'm in a tunnel. I'd rather have a client-side rendered UI that keeps my context and tells me "Sorry, try again when you're connected again".
Even better if it works fully offline and syncs the transactions I've done with a backend once I'm connected again.