Most of what you said applies to the Linux kernel too. It's good to have other options, but being popular does not mean something is bad.
LinuxSBC
The problem is that breaking it will also break a lot of Apple devices.
I like bottom right the most, but it does't really feel like a default wallpaper as much as top left. Middle right feels like part of a tiling WM with custom colors more than a default for a DE.
People try to upload CP and get the admin arrested for hosting it.
He's pretty much the quintessential QA tester. He wants to do things his way, regardless of whether or not the OS wants him to do that. He's usually skilled enough to fix anything he messes up, but he doesn't know enough about Linux to do that, so he ends up breaking things. I feel like most people have a better experience than he did, but his technique uncovered a ton of bugs and usability issues that significantly improved the Linux desktop to have fixed.
Tiling addons. I like having a full DE, but I also want tiling, so Pop!_Shell on GNOME and Polonium on KDE are invaluable (and yes, COSMIC looks really promising).
Thanks, though your correction is also incorrect. Display managers, like SDDM, GDM, or LightDM, are the login screen. They're called "display managers" for historical reasons, but they also run on top of the display server.
It would be largely fine, but be careful. Being immutable, a lot of things that you would expect will work differently or not at all. I would not recommend it, but if you're in for a challenge, it's not bad.