IMO “default” has to be simple and user friendly. I’m not saying kde is not user friendly but it has overwhelming amount of options which I personally think is too much. of course there are people that use these but i think something like gnome is very simple and more approachable. I think this is why many distros use gnome as default. I would consider myself to be a mid level user i started with mint linux -> ubuntu -> debian -> manjaro -> garuda -> arch -> fedora -> and finally settled on nixos. I have tried lot of desktop environments and i liked kde very much but the amount of customisation made it a chore after a while. i can use the defaults perfectly fine but if something has a toggle to choose different options i try all of them. Which i agree is a not applicable for everyone. But i think sometimes taking option away from user works example ios and macos. So, i settled on gnome. I understand it is a personal choice. I’m just trying to explain why it might not be a good default from my perspective.