The concern was actually more for the cars interlocking wheels, and so Dallara added the fin behind the rear tire.
I think one reason the rear pods are going is that they tens to be easily knocked off, leading to large debris on the track.
Went to the Dallara factory in Speedway, Indiana today for a tour. We were not allowed on the factory floor however because Dallara was constructing aero test pieces for the 2018 cars and did not want them viewable by the public.