I have already suggested to @Mermoo that drag factor would be the unlikely reason for restricting the option of a sun roof on the e.Golf.Hi MERMOO THE REASON
The lack of sunroof is not drag related at all, it is about the weakening of the structural integrity. There are reports and pictures of cracks running from the sunroof aperture to the nearest seam, that is why. Also when a sunroof is closed how is there any drag? There is other reasons, the roof when opening, catches on the headlining and pulls/rips the headlining, this maybe as with other cars that the sound deadening drops down a fraction (poor quality glue, plus the suns heat) and the roof catches and so as it goes back it can do damage. I used to repair the odd one or two from different manufacturers 20 yrs ago, I could do it without taking the headlining down. The pictures of the roof cracks I have seen, as an ex MOT tester I find shocking, there is a major problem here in the very structural design of this car, I wonder when they had the crash tests did the cars have sunroofs fitted? So forget the excuses and realise this company is as there CEO had to admit "A dishonest company". I started as a mechanic in 1969 and I have never seen such cracking, in any other cars roof at all! To qualify this statement, in the late 70s- early 80s there was fashion to fit flip sunroofs, they were cheap and we fitted a lot. This required cutting a hole in the roof with a jig saw but I never saw any of those cars come back with cracking in the roof panel. I shall leave it at that. If you don't believe me google something like Golf mk7 faults that should bring it up. The general car mags wont touch this flaw, VAGs advertising arm would never advertise with again if they did.
Removing that much structural strength from the roof of a modern car today, then increasing the weight of the car by installing a heavy battery pack must compromise the over haul upper strength of the body shell.
You could chop a big hole for a sun roof from a car in the 60 and 70’s because the metal sheet used was much thicker than the cars today.
The strength comes from the shape of the pressing in a lot of modern cars.
The actual steel they use to press the panels, is wafer thin today !.