I think it's worth reproducing Ton's statement on this issue here:
A lot of (really great) UI design proposals that went on the past week give quite a false impression that it’s either feasible to just replace the UI, or that its possible to pay for it – even when it takes a dozen people a year to work on.
The misconception here is that the “UI” is some kind of separated entity of a program, which can just be peeled off and replaced with another interface. However, a really good UI (and certainly Blender’s) is not just an abstract independent layer, it’s the reflection (and sometimes even the core) of the program’s design.
You can easily state that a significant part of the Blender code (more than a quarter) is “the interface”. You cannot code any tool without understanding “the UI”. You cannot work on compositing or rendering without keeping track of it. A particle system is “UI”. How animation systems relate is “UI”. How files read and write is “UI”. The way how undo works is even “UI”.
So – replacing a UI can be as impossible as trying to replace the script in a film. Let’s not go that way now. A full rewrite of Blender might happen once, but I think we have enough quality code design in Blender still – and a whole lot of open todo topics – to fill at least several years of coding time with.