> This distro doesn't seem to be born out of some real need for non-KDE-developers?

It's born out of a few things:

a) KDE as a community has increasingly focused on good and direct relations to end-users of late, which e.g. has resulted in most of the funding now coming from individual donors. Wanting to make more of their computer-using experience better isn't a strange impulse to have.

b) The community has hardware partners (e.g. laptop makers) that want to collaborate on pre-installing something with a KDE-focused out of the box and user experience. That has so far been Neon, which has a number of engineering and stability issues that have been difficult to overcome. KDE Linux is an attempt to improve on that.

c) It's also generally driven by a lot of the lessons learned from the SteamOS and Neon projects, and is attempting a lot of new solutions to risk-free updates and hackability, oob experience, and down the road likely also backups. The team does think there is a value prop to the distro as such beyond the KDE GUI.

d) The developer audience isn't unimportant either. More KDE developers on an immutable+sandboxed apps distro will mean more eyeballs on e.g. Flatpak problems, improving that for everyone else. Many recent new distros that ship Plasma by default (e.g. SteamOS, Bazzite, CachyOS, etc.) benefit.

So..

a) I get that a lot of users use KDE. And they love the Desktop Environment. But is there demand for an OS? Would those users switch? I hope so, but for such a big decision to build, support, maintain a whole OS, i'd expect some kind of poll maybe? Some input saying "30% of KDE users would switch to KDE OS"? Is there some kind of proof? I've been using Gnome OS for years but never felt i would want to switch to some Gnome OS. The Desktop Environment is one of many tools in my distro (for me, at least).

b) Supporting lots of hardware (expecially Laptops!) seems to be a huge time sink for people not primarly involved in kernel/driver stuff, or not?

c) ok..

d) Same as a): Will all KDE devs use KDE OS? And is it good to have the KDE Devs use KDE OS, when the majority of users use Arch/Debian/Ubuntu/Fedora? I'd rather have a good chunk of those devs use my distro...