Agreed that a taller curved window would be more expensive, and these usually get replaced with something cheaper (vis. Class 309 Clacton Express windscreens).
One train of 1938 stock, was I believe, experimentally given taller windows.
Having said that there is no reason why the existing window in the mock-up should have another flat window above, angled to mimic the profile of the car.
If these are meant to replace the 1973 Piccadilly line stock and the remaining 1972 stock, then significant portions of the line are in 'open air'. So for standing passengers there is some relief from staring at the same advert for the whole journey - unless there may be a loss of revenue from the adverts . . .