This is why we still need html/css for interfaces, so the PWA with wasm still seems to be the best way to distribute an app that works anywhere, or PWA for mobile and electron for desktop with wasm enabling very high performance client features.
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?
I had to laugh at this.
I am writing a PWA now and this is definitely an issue in "desktop mode".
The thing is though, with giant monitors (I develop on a pair of 4k curved ultra-wides) there is so much room that most non-game apps just don't need all that much space. If you do use it all, it becomes cumbersome to move the mouse all the way to the corners because it is so far away. And even if you did use it, there would still be acres of unused space most likely, white or some other color.
I suppose you could do a true re-write and put multiple mobile screen onto one desktop screen, but that would be a very heavy lift and in the case of a saas app that will largely be used on mobile, it is pretty easy to understand why it never gets done. Bad cost-benefit and it would delay your launch.