Overhauling the GIMP UI would take way longer than three months of a single UI designer. As an operation, we are talking UI/UX design, implementation, testing, and not to mention having to refactor existing code such that it can actually handle such a radical UI change.

I'd put the (very fuzzy) estimate at something like 2 years of consistent (full time) effort from a team of, say, 5 people. That puts the cost of such a change closer to a million dollars.