About the plugin part - to be fair, and as far as I know, it is not a "plugin" in the same way of, say, Gnome extensions - I guess OP meant more than it is a module. It actually comes by default in every Plasma install.

And though am pretty sure you made that 0.1 statistic out of thin air, also can bet if the 'one wallpaper per virtal desktop' part were working, a bigger percentage of people would be using a solid background color. Maybe even more than the people who would be using one wallpaper per virtual desktop.

I can have different wallpapers for different activities. Why can't KDE have different wallpapers for different desktops? Is this just a missing feature? Perhaps it can be re-enabled by a Plasma Applet?

When you need to have different options showing complex things as a wallpaper, it's best to factor them out as plugins, or at least separate implementations of some interface in their own files. When that happens, why not also use that interface for the basic functionality, ensuring a common code path? You don't want "enum WallpaperStyle { Image, Color, Plugin }" when you can have just "WallpaperPlugin".

Sounds like an abstraction not far enough: if you want that single-color optimization, make it an image format that only allows 1x1. It would work whereever a reader for that image format exist, but code that's eager to take performance shortcuts would be free to add a little optimization. Same for the configuration UI, an "image or color picker" would be fully transparent to image picker only.

Bonus points if the specification of that image format allows itself the impertinence of annexing the file name: "if the file name starts with a hash sign and is followed by three or six hex digits, the color of the pixel is that color, contents of the file are considered a comment"