The sonar part is the restricting one. With optical flow you would not be limited to a certain altitude.
I have looked at sonar sensors, and they get significantly large for longer distances. Even the larger ones are limited in range, max being about 20 meters. On a drone you have other problems with sonar use: the noise of the props can significantly reduce accuracy and sensing capability. Other ways of doing object avoidance would be to use lasers for measuring distance, but there is another set of issues with that technology as well.
It will happen, it is a matter of evolution. Not on the inspire 1, but definitely in future models.