Why are movable aero devices banned?

[Tim.Wright]

Cars could become too fast very soon.

Devils advocate here... What's too fast????

I'd say the speed at which the track and car based safety features become ineffective.

Run off areas, crash barriers and impact structures are designed on a criteria which prescribes a limit decelleration from a defined starting velocity. If this starting velocity increases then your existing tracks and cars are no longer suited for the job and need a re-design. Easy for a car, difficult for a complete race track.

[Just_a_fan]

Cars could become too fast very soon.

Devils advocate here... What's too fast????

As TimWright says, the point at which the cars carry more kinetic energy than can be reasonably dealt with by the car's / track's safety systems. We all know that kinetic energy increases with the square of the speed, so adding a few tens of km/h to the top speed (or even peak cornering speed) could be enough to make a currently survivable accident in to one that injures / kills a driver (or someone trackside).

We can deal with some of that by making run off areas deeper, adding extra layers of energy absorbing materials in front of the barriers etc. But what if this isn't possible at some tracks? Do we lose circuits? Do we limit the cars at those tracks? Do we just say "tough, sonny, go out there anyway"?

[netoperek]

Cars could become too fast very soon.

Devils advocate here... What's too fast????

I've seen an Adrian Newey quote once, more-less saying that if there were no rules at all in F1, cars would be way too fast for any human being.