Odseus said:
It makes me wonder if wow would be as popular as it is today without the customizeable ui. Or would blizzard have picked up the slack and actually fixed their UI so that it doesn't look and function like ass.
The real trick is that everyone likes their UI a bit different. Ask a dozen World of Warcraft players and they'll probably all like something a bit different. Some people thrive on additional information: you'll see a screenshot of their UI and it'll be full of boxes, bars, numbers everywhere. Even within this group, the presentation and arrangement of that information changes. On the other hand, some people just want something lightweight so they can focus on actually seeing the boss instead of seeing a bunch of boxes.
So suppose you're Blizzard, and you're trying to make a UI that's all things to all people. The first group will want to have all of those boxes available to them somewhere, and will want to be able to make the boxes look and act different, but the second group really doesn't care, and in fact might react against the clutter. What you're looking at is a fairly horrendously large system, and a fairly horrendous configuration menu. (Incidentally, that's one of the knocks I remember hearing against Discord: too much configuration.) It's smarter to just build in a plugin architecture of some sort and let the modders go from there; that way, if you want something more complicated you go and find it, and if you don't you can use the basic UI.
I'd honestly be surprised if anyone built a major MMOG nowadays without some sort of UI plugin ability.