Part of the problem with opening DF up to allow the design of custom UI is that the game is constantly changing, sometimes in pretty profound ways. Each third-party application project requires a good deal of maintenance, but none of them represent a full custom UI. A full UI would require a great deal of maintenance, but a lot of the earlier work would become obsolete later down the line. That's not necessarily a prohibitive factor, but it means a lot of work would go unappreciated; and I don't think anyone wants their hard work to go unappreciated.
When DF becomes near to being 'finished' that would be a better time to implement a streamlined UI. If Toady wanted to earn more money, he'd focus on attracting lots more users to his site, and focus on improving DF's user-friendliness. He doesn't seem to want to do that. We all like DF as it is, otherwise we wouldn't play it. So who really benefits from improving the UI now anyway? We're happy, Toady's happy, and non-dorfers are happy, because ignorance (of DF) is apparently bliss.
Believe me, when DF is finished in 2026 and people are playing it with their psychic probe interfaces and streamlined Web 5.0 graphics, we'll enjoy the smug satisfaction that we soldiered through with the confusing UI we have now, we few hypernerds dissecting the arcane glyphs and convoluted menus with gleeful abandon.