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.
1. Personal computers before the 21st century were really kind of shit. Let alone mobile devices.
2. Software was largely a product that people paid for. It even came in boxes.
3. Software vendors were usually in a highly competitive environment. They had to deliver value for money if they didn't want to get eaten alive.
This meant that the software had to both work on the limited resources of 1990s shitty computers—limited storage, limited speed, limited display colors and resolution, etc.—and be useful to the end user. So companies were kept a lot more honest in terms of UI design. Circumstances forced them to deliver functional, efficient UIs. These days, our computers are fairly powerful and companies are in the business of selling services (or eyeballs to advertisers) rather than software. The user-facing software itself is a loss leader, and if making it a shitty Electron app, or desktop-mobile "convergence", helps save development costs, companies will do it.