that's been KDE's driving philosophy for years
Yeah, but KDE is not Linux, and Linux doesn't know where that driving philosophy hides. Linux is KDE/GNOME/Whatever
it's a good thing, that proprietory vendors have a cost increase with every additional distro supported while free software developers get pretty much all distros for free
Please explain how proprietary vendors have to abide by different rules than open source developers please. Also, your statement is quite obviously false since the vast majority of software vendors are entirely un-interested in developing for Linux.
The latter part of your infantile rant is also proven wrong by the fact that statistically ZERO vendors develop for Linux. I have zero problems developing for, on and under Linux, and I have been doing so for years. In fact, I was part of a team that delivered one of the first commercial applications written in Java, running mainly on Linux (and SunOS and HP-UX) way back in the late 1990s. The fact that there are no (statistically) commercial apps for Linux shows two things, one of them mirrored on Android: 1/ Developing good UI apps for the Linux platform is hard, and it is difficult to chose a technology that will be consistent and functional over time. 2/ Linux users don't buy desktop apps (Android users are far more reluctant than iOS users to pay for apps).
Consider someone like Adobe, they have a vast array of industry-leading apps, they are functionally identical on Windows and on OSX. You can be quite sure that Adobe has a clear separation between application logic and display logic, otherwise the apps would end up having far more issues that were platform specific than they do. This means that the majority of something like After Effects is already mostly platform independent. Porting from OSX to Linux would be anything but trivial, but a reasonably cost-efficient project. If there was a customer base. However, they would have to chose between (for example) KDE and GNOME. The problem is that one year KDE is the hottest thing in Linux land the next GNOME is. Who knows which is next. This drives up maintenance cost for a platform where nobody buys software. Business wise it would be idiotic for Adobe to go that route with anything marginally more complex than what they have on mobile.
If anybody in Linux land ever cared about the desktop, there would be a single UI platform with a consistent and long-lived API. Nobody cares about that though. This is why Linux is a (great) server OS, a mediocre desktop OS, and will never conquer the mainstream desktop platform. Ever. OSX won the Unix on the desktop war, and if you need a decent Unix on the desktop for running something that is not developer tools, you'd be insane to chose anything BUT OSX.
End words: I have been using Linux to develop cross-platform apps since some time in the 1990s. After moving into the mobile space I was "forced" to get a Mac since mobile today == iOS (Android users don't pay for apps) and you are basically required to have a Mac for iOS development (has recently changed a little, but that's another matter). OSX is what Linux could have been 10 years ago if someone had ever cared. Nobody ever did, and today there is only one rational solution for Unix on the Desktop.