“What is dangerous about flying 150 feet above the highest obstacle?”
If you have an emergency, you don’t have enough altitude (and time) to react and perform emergency procedures. Refer to the recent US Airways ditching in the Hudson River. The guy was climbing through 3,200 feet when he hit the birds and his engines flamed out. That barely gave him enough time to find a spot to put the plane on. That was a small Airbus. A gigantic 747 at 150 feet AHO does not stand a chance if its engines quit.
In the Army we did a lot of low level and nap-of-the-earth flying. But that was over unpopulated areas. A heavy airplane crashing somewhere over lower Manhattan would be - and has been - a catastrophe.
I did a lot of low level flying as well. By your standard, there would be no urban airports. The odds of AF1 losing all 4 engines is...remote.
From what I saw, most of the flying was over water. If it HAD lost all its engines, it would have ditched in the water.