To be fair re: 2D buttons and the like... I also hate them but I can see why the world has gone 2D.
Love it or loathe it, the new standard for computing is smartphone. Most users of any kind of computers are content consumers and they want selfies, whatsapp, Facebook and... it pretty much stops there. On those small form factors with limited screen and only touchscreens to work with flat, 2D buttons are rather required.
Users also now EXPECT such on their desktops because the desktop interface is now rather driven by the smartphone! I live in SE Asia where smartphone rule more than in the West. People who can't afford a laptop, even a cheap ASUS netbook, will have a smartphone.
GNOME have clearly gone all in with the smartphone and seem to take the view that *like Windows 8 assumed you were using a tablet,) you expect to use your laptop the same way as your smartphone - a few buttons, touchscreen everything, no configuration, run some apps... end. Hence their policy these days of "If in doubt, cut it out." I also think, though this is conjecture, that Gnome are taking a policy of making the UI as minimal and, thereby, as light as possible so as to be used on smartphones with minimal load and with anything pertaining to the desktop being for 'experts.' Every feature Gnome has cut out I understand can be restored by installing an add-on. So it seems to me that Gnome is moving towards base GNOME being just a minimal UI for smartphones ONLY by default and 'expert' users installing whatever add-ons they like for their desktops.
I figure GNOME has lost interest in the desktop have left it to the user to sort out Gnome for the laptop. Only, cutting features out does seem to endear distros and more and more are turning to Gnome 3 by default! It seems the 'if in doubt, cut it out' approach has traction with distros!! [I guess no options means no config issues equals less support hassles... a very Mac approach.] More and more distros are dropping KDE and going Gnome. I love it and won't use anything else but I keep seeing "New distro - running Gnome 3, while older distro = KDE dropped..."
I can see why Neon came to be.
KDE, though, is doing the same thing a different way. They are keeping the desktop experience but also want KDE on smartphones and so working on convergence which is a risky game given NOBODY has succeeded with this as yet but... KDE has spent serious time building up to this and Plasma will operate in different modes depending on the device it is running on. I think that is smart and the first plasma phones I think are coming out next year. But... there has to be some common look and feel and that means... flat, 2D buttons.
I also think KDE has got to smartphone to gain traction. Outside of servers and niche content production bases the world has gone smartphone! I can see Neon being developed with smartphones in mind. Like Gnome it's a minimal install that you add to - but can still be used as a desktop UI by default in a way Gnome really cannot any more... unless addons are supplied.
Whether we like it or not, the world has standardised on smartphones and users expect buttons and scroll bars to look like they do on their smartphones and the industry has had to bow to public demand. I don't like but that's the way it is. BTW, this also explains the crude, clunky style of the slowly-becoming-usable 'Discovery'... it is clearly app designed to run on a smartphone first. The smartphone rules now baby!
For the record, I don't have a smartphone. I hate them at multiple levels but my wife has one and my clients will ONLY respond to whatsapp and nothing else including email and phone calls!