Personally, of all complaints, I don't understand those against the interface. It's way more comprehensible now, and less off-putting to newcomers. I have to say that when I went from civ2 to civ4, I had trouble understanding what everything meant for a while, and I actually had already seen other people playing it!
Sure, the ads on the right of the screen could use some work as to avoid the spam of useless information, and tell you other things that might interest you (maybe even have an option screen to choose your preferences), but that's about it.
And on Civ5, the game is far from dead. Most civ5 players would buy an expansion with their eyes closed, should it add gameplay options (as opposed to extra civs, for example); I don't think 2K games will ignore that.
EDIT: plus, what Putmalk said, obviously.