Would I be sad if they removed or broke mechanic addons, like meters or boss timers? Yeah, but I wouldn't quit. Would I be furious if they removed the ability to customize how I receive the default information in the game, like buff/debuff durations, health, resources, and placement for the elements of the UI? Yes, and I'd probably quit. I've tried on numerous occasions where am I without my home computer, to play wow with no addons. It can't be done, not with my muscle memory and habits. Mounting is handled by an addon and a single keybind. When I'm elsewhere I tend to bind it to a single mount that works both flying and ground. Why have 150+ mounts if you're not going to see them? Maintaining long-term buffs like fort? Hell no, I'm too much of a space case for that, plus I refuse to bind buffs. I've used SmartBuff since TBC. I play on a 42" TV, and having my health and target health in a completely different part of the screen than my character and abilities is impossible. The default UI is like... really crappy too. I mean it's a particular style, which is fine, but the bars are tiny, the textures are terrible, the text doesn't fit the frame, and the way buffs and debuffs are tracked is abysmal. It's much easier to watch a bar run down or a number tick down that it is to watch a darker texture sweep it's way around a square that's like 15 pixels wide.
Nevermind model revamp, WoW needs a UI revamp. It worked well enough when we were all on CRT monitors and 1024X768, but I haven't had anything below 1920X1080 for like 5 years now. I use ElvUI, but I despise the "futuremodernallblack" look. If Blizzard allowed customization of spacing/location/size and text I'd probably work with that so that I still felt like I was playing WoW.