Yeah OBVIOUSLY flat UI was a huge mistake, it's not every single other UI design in the last few years has adopted it. Personally I FAR prefer the flat UI stuff than the awful skeuomorphism design language we had before.
I don't get the hate for the touchbar either...like it makes an area of the keyboard that was effectively useless for the average person into something that does something. What's so bad about it?
Only reason one more thing had an impact before was people weren't nearly concerned with Apple rumors and their supply chain back then and their operation was smaller so there were less leaks. Were Steve Jobs around today we would still have just as many leaks and the big announcements would still be knows weeks in advance. Part of the territory of being this successful.
The primary reason other UIs have adopted it is because Apple did it, so it must be trendy. Don’t mistake success with actual good design. Honestly, take a look at Android’s Material Design. It’s aesthetics are quite nice. There used to be an option to reintroduce button outlines on iOS, but Apple broke than in iOS 10 and now just underlines the text you can click like a 1995 html link.