1. Some day, those users think "Hey, I'm not happy with some setting, what do I do?" and they can do nothing.
2. The users who need more functionality can't get it - and feel like they have to constantly wrestle with the app, and that it constantly confounds and trips them up, even when they express their clear intents. It's not like the GNOME apps have a "simple mode" and an "advanced mode"
3. The GNOME apps don't even really go along those lines. You see, even non-savvy users enjoy consistentcy and clarity; and the GNOME people have made their icons inscrutable; take over the window manager's decorations to "do their own thing"; hide items behind a hamburger menu, as though you're on a mobile phone with no screen space; etc. So, even for the non-savvy users - the UX is kind of bad. And just think of things like the GTK file picker. Man, that's a little Pandora's box right there, for the newbie and power user alike.
One could say the same about people who don't bother to learn ffmpeg CLI.