Well, the bigger problem that people who don't program UI stuff don't realize about UI stuff is that it is actually hard to program. The main culprit here is that input is different on different systems, and while a lot of problems have been solved in the past, everyone who makes a new UI toolkit (like toady is doing, to keep ascii compatibility), needs to solve each of those problems manually. You have things like some mice only reporting button-presses but no button-releases, hover states being hard to track, having incorrect timing, etc.
On top of that, from the talks, it seems DF is going to have a responsive UI, which is something that the majority of games aren't doing (though, that might also be that unity doesn't do responsive UI well? Not sure).
Like I said, I am pleasantly surprised DF is going to become more simcity like in it's UI, because with my experience of UI stuff getting to this point is already really hard.
Furthermore, the zones stuff you are talking about isn't just interface, but it is also something that directly influences how the AI works with these elements. So it would require another rewrite on top of the functioning UI framework being build. EDIT: and, come to think of it, will destroy backwards compatibility.