I wouldn't argue that Linux on the desktop is the pinnacle of user experience, but it's pretty good these days. I personally run Xfce as I have since the 00's, and over the years it's become rock solid. It's also simple, does the core things I need a DE to do, and that's all I want.
I think the current relatively decent state of DE environments speaks to what volunteers can accomplish. Are there better UIs in the market? I'd argue that Apple is doing a better job than anyone here. But the Linux options these days are firmly in the "good enough" category, especially when the alternative is the increasingly user-hostile Windows UI.
I think the current relatively decent state of DE environments speaks to what volunteers can accomplish. Are there better UIs in the market? I'd argue that Apple is doing a better job than anyone here. But the Linux options these days are firmly in the "good enough" category, especially when the alternative is the increasingly user-hostile Windows UI.