For many years now KDE has focused on polish, bug fixing and "nice-to-have" improvements rather than major redesigns, and it paid off.
I often wondered why desktop UIs became so terrible somewhere in the 2010s and I don't want to attribute it to laziness, greed, etc... People have been lazy and greedy since people existed, there must have been something else. And I think that mobile is the answer.
UI designers are facing a really hard problem, if not impossible. Most apps nowadays have desktop and mobile variants, and you want some consistency, as you don't want users to relearn everything when switching variants. But mobile platforms, with their small touchscreens are completely different from desktop platforms with their large screens, keyboards and mice. So what do you do?
In addition to mobile, you often need to target the browser too, so: native desktop, native mobile, browser desktop, browser mobile. And then you add commercial consideration like cost, brand identity, and the idea that if you didn't change the UI, you didn't change anything. Commercial considerations have always been a thing, but the multiplication of platforms made it worse, prompting for the idea of running everything in a browser, and having the desktop inferface just being the mobile interface with extra stuff.
You keep the UIs separate. Dumbing down desktop UIs to mobile capabilities is just as bad of a design as it was when people tried to jam a desktop UI into mobile. You have to play to the strengths of the platform you are on, not limit each one based on the other. Yes, it's more work, but it's well worth it to have a product which is actually good.
The insanity of it is that many websites push their mobile apps to use them. So, you get shitty mobile sites that ask you to use their app on mobile and are bad on desktop because of the stupid development philosophy (including poor information density and oversized interface for big touch targets).
The whole point of the first iPhone web browser was that you could actually use most typical websites without any effort on their part and it was good enough. Because of the display size and navigation effort required it wasn't the most confortable but the more time passes the more I believe that was kind of the point and almost a "feature" in itself.
We got there because people are glued to their phone, and sadly it's not even a good tool for efficient web browsing (it's useful for quick information gathering but that's it).