i wonder if todays rear bumpers are just to low for cosmetic reasons.

on my car the rear bumper has quite a lot of ground clearance, you can see most of the exhaust and the spare wheel well, which doesn't look to (aerodynamically) clean, but at least there's no big plastic scoop behind it to cover it.

its to much of a mess to try and cover it, but at least the messed up air has a clean exit path.

smoothing things out is good, but whenever possible removeing restrictions might help.... as far as aeromods go, i'll usually do whatever's achievable even if in theory there's better solutions

Amoung the worst of these are the Last generation Firebird / Trans-Am / Camaro, as well as the late '80s- early 90's Mustangs.
Their bumpers actually hang quite low into the airflow and have quite a pocket to trap the air.
Look at the Pontiac G6. It is a beautiful design, but the bumper actually looks as if it is inflating from all of the trapped air beneath the car.