Think of it as the product of the new generation of techies who had their first contact with computers after Mac OSX was released, and now all they can/want to do is emulate Apple and their HIG.
I'd be inclined to agree with you, but it's hard to me to believe that all these kids went to college (or got some type of computer training) only by working with their phones/tablets.
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!)
GNOME is primarily about making Free Software that is highly accessible. The desktop and GTK toolkit both predate OSX, so I'm not really sure what the purpose of the comparison is.
This criticism is for the more recent versions of GTK.
Killing theming and removing features when they do not have a serviceable usable alternative does not make it more "accessible", it is just the standard "we know it better because we study the behavior of a large group of users" excuse that they give to treat it everyone by the lowest common denominator - just like Apple.
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.