It's not that they only use their phones/tablets, it's that the phone/tablet is the first tool they reach for, the default device, and UI conventions from that space feel correct and natural to them, just as they feel clumsy and jarring to me. (How old am I? Old enough that I still use an opto-mechnical mouse with a ball, because I still like the extra weight and inertia it has over a pure optical mouse!)

The youngest had to be taught, as part of intro to CS, how files and directories worked.

Such a person may have used a desktop, but desktops are foreign to them.

Yeah, that much I generally agree with. Many of the 'opinionated' decisions that came with GTK4 are either regressions or strange and inflexible.

That being said, I will shamefully echo the words of the GNOME maintainers; their goal was not to "kill theming". They wanted to stop distros from shipping themes by default, which was arguably just as deranged but at least somewhat understandable. Nowadays we have stuff like Gradience too, which lets you theme LibAdwaita apps. You're still forced to use the ugly old Adwaita buttons, though.

So... make of it as you will. I left GNOME completely after the GNOME 40 update, I sympathize with people who hate the current desktop. The toolkit itself is "fine" in my unprofessional developer opinion.