[edit] for my part, as a Mac user, I couldn’t tell you whether my desktop’s full of icons (probably not?) nor what my background image is (… the default? Which I’m guessing is probably a mountain or a wave or something?) because I see my desktop so rarely, and never pay any attention to it.

I get it, but 100% of Mac users I've helped in my 25 years of professional computing had messy desktops. That's dozens (maybe close to 100) of Mac users. Not scientific, not conclusive, but it's exactly 100% with no exceptions. This includes fellow IT professionals and college acquaintances. The most recent experience being two days ago, where I assisted a Mac user in changing email settings. It's also very common for these Mac users to not understand file/folder structure. I often guide them to download a file/app, and once downloaded they generally have no idea where it went unless its... on their desktop! Same for "open" applications. For PC user's I'd say it's in the 10-20% range, low enough to be unremarkable.