If the active aero adapts behind another car, then naturally it has to adapt away from the otherwise ideal configuration, no matter which way you cut it, it's compromised. Maybe less than currently, but with smaller engines and less power it'd be even more of an issue.
Nobody would make a mistake because with more aero downforce for more grip in the corners (as you yourself stated in your OP, if we're going for the 'who mentioned more grip' angle again...) and even less power and torque they'll be even further inside their traction envelope than they already are due to lower acceleration forces and higher apex speeds.
The series doesn't need a challenge, or want a challenge, or have a viable reason for giving itself a challenge. The series just needs to entertain it's audience and make money.
The engineers and drivers need a challenge, not the series.
What the series needs is less aero dependence and more on the tyres and chassis. Your tyres and suspension/chassis don't generally care if there's a car in front or not.
Frankly it's been nice to see the engines make more difference for a change rather than someone just buying the best aero team.