Open-source desktops are never perfect for anyone, but everyone expects them to be. For everyone who says they are too bloated, someone else says they are too spartan. That's probably why there are so many of them. I would expect working on Gnome (or KDE or Cinnamon or Mate or XFCE or LXDE, the list goes on) to be a thankless task. You try to please everybody and yet please nobody. The imperfections of Linux desktops…
And UI/UX designers work happily and feel welcome on Windows and Mac OS systems, which are what everyone non-technical buys anyway.
Could you elaborate on what you mean by "feel welcome"? AFAIK neither of those systems is open to contributors: if a UI/UX designer wanted to improve some aspect of the design, they're either limited to whatever config/theming the company decided to allow, or they must go and convince the company of their idea (especially sales, marketing, etc.), sign NDAs if they want to see/fix any of the implementation details, and so on.
With FOSS they could just post to a mailing list or something, which I would imagine is more welcoming. Maybe the difficulty is social: FOSS projects certainly suffer from toxicity, but their transparent processes also make it more visible. Companies are better able to hide toxicity using firewalls and private/internal communication channels.