I disagree with the whole premise of having a single app for both desktop and mobile. Even if you give developers the tools to make a good app for desktop and mobile, they won't. They'll make the mobile app first, ensure that it runs on desktop, and then dust off their hands and call it a day. To quote the Purism blogpost cited elsewhere in this thread:

Web designers now have toolboxes to design web pages, which they adjust for mobile or desktop in order to get easier readability and use.

Except, web designers don't adjust web pages for mobile or web. They adjust pages for mobile, and then what you end up in a desktop web browser is acres of white space and buttons that are the size of your head.

The way I see it, this is Linux UI framework developers chasing Apple, Microsoft and Google's taillights yet again. Sure, Apple, Google and Microsoft don't care, but that's because they're trillion dollar companies and they have to go where the majority of the customers are. But why does Linux have to go there too? Why can't Linux UI framework developers focus on an under-appreciated niche (desktop "power" users) which are increasingly neglected by the megacorps?