If GTI is less popular than GTD
Sounds to me like you have spent a hell of a lot of time and effort in this thread convincing yourself of this. Buyer remorse? Secret GTI fancier? I think so.
Nope, just helping someone consider all of the pros and cons - singularly they are nowt, but accumulatively they are significant. It's easy to be looking at the GTI right now after ordering the GTD as there are only GTIs to look at in the showroom right now and I can take a look knowing that it looks the part and I can visualise my GTD looking the same except for the red embellishments that I really don't care for.
I will get as much enjoyment from a GTD as I would a GTI, the everyday driving experience on UK roads will be largely the same as I don't partake in traffic light drag races but do like the overtaking ability that both cars have.
I currently enjoy my 170TDI Scirocco (I had a test drive in a 2.0TSI before I bought it and was not fussed for the extra performance vs running costs, and the GTD is closer to the GTI than the 170TDI was to the 2.0TSI Scirocco) and an extra 14PS can only add to that enjoyment.
I like visiting the pumps twice a month rather than once a week - it'll pay for an extra week's holiday somewhere nice.
If VW wanted the GTI to really stand out, they wouldn't have made the GTD to the same spec.