I think a major point of importance for a project is to attract new people in. The first time I opened FlightGear as a 16-year-old, I felt had this sigh wondering what antique stuff I had downloaded: It's first impressions.
The Flight Simulator X interface had color, responsive design (and cheesy music), and the 12 year old in me I appreciated that. I have countless friends who still spend copious hours making liveries for that use with that program- imagine you could channel that energy to FlightGear instead...
I don't subscribe to Hooray's point that the project is already too popular, but...
It's a question of allocating time. I can spend my time coding things that actually improve the sim, rather than polishing the gui. You're arguing that if we would have a nicer GUI, we could ultimately attract more people contributing. Well - maybe. My personal experience is that arguments like 'if we had X, Y would happen' are quickly made. I have coded a few X since I am with FG, and Y has almost never happened as a result.
The people who contribute to FG substantially and in the longer term are probably the tinkerers - they're more hooked by seeing what is all configurable even on the xml level than by a polished GUI.
Or, to turn the point around - with things you can learn in a week about Nasal and canvas and some help from the folks who maintain it, you could make the gui you want - or join one of the ongoing efforts. You're not stuck with complaining about it, you can just go ahead and learn how to change things.