> how many companies could actually do this before cascading collisions starts becoming a real worry?
Twenty of them at 100,000 birds each to start approaching the density of planes in the sky [1]. Not around an airport. In all of the sky. Oceans and all.
Practically speaking, this is not a pressing concern for our generation.
Speed matters a lot. You can fit a lot more walking people than speeding motorcycles in the same space.
Satellites need to travel at 8 km/s to not fall down.
It's interesting that people have a hard time visualizing this. The area in Earth's LEO is, definitionally, bigger than the Earth itself.
The SEA parking garage fits 12,000 cars in it. Two of those spread over the entire planet would be an imperceptible amount of space. You could drop a pin on a map your entire life and probably never hit one.