Can I interest you in a nice flat in east London?
No need to sell the helipad, just sell the helicopter and put the AA gun on the pad itself.
As far as the dedicate altitudes thing goes, that’s already done, and pilots are still trained to keep their eyes out for other planes that they might smack into. Only takes one distracted pilot or a faulty altimeter to ruin the morning commute.
Also, when cars crash into each other, they crash into each other and not a whole lot else. When aircraft collide with each other, they also tend to crash into whatever is below (and around) them, sometimes for a good distance depending on the speed and altitude involved.
As far as parking in apartments and workplaces goes, hopefully the helicopter can fold up into a briefcase and placed on a shelf on your way in. 
Helicopters are noisy.
One of my neighbors has a Hughes 500 on a helipad. (It’s built on top of his boat dock.) I can assure you that the sound of even **one **helicopter clattering away in the morning is annoying. A whole neighborhood full of them would probably lead to violence.
Kind of related, but what happens if one helicopter flies directly over another? Does the one on the bottom get sucked into it due to the downdraught?