Re: And if you think about it for more than 10 minutes...
Currently space is not an issue because there isn't many of them, get 2 million of them up there each needing to be at least a mile apart from each other and things change.
Also Air traffic control is stressful with the current setup, imagine people having to organise that many flying vehicles. Yes I know they would intend to automate it, but that is not currently the case.