How old is your wife's GTD?
If she bought it a couple of years ago, way before the diesel issue and way before the Golf 7.5 was on the agenda she would have paid pretty much full price. At that time Golf GTD were in demand so dealers could get top price for them.
Today, there is a glut of MK7 GTD available, there is a new model on the horizon and VW reputation has taken a beating.
I agree to some degree about your comments about the dealer but you cannot get away from the fact the market for GTD is now totally different and trade in prices reflect this.
Many people on here complain about hideous depreciation but they seem to loose sight of the fact the 20% of a new car price is VAT that belongs to the government, the car was never truly worth the price paid in the first place.
VW are dropping prices of all MK7 to clear stock ready for Mk7.5 its therefore inevitable (to me at least !) that trade ins will take a proportionate hit too.