If a startup were able to truly solve the first two issues alone, they would not be burning those world-changing engineering solutions on flying taxis.

I don't know if a silent, fail-safe, and efficient method of flight is physically impossible or not, but I do know this is low on the list of applications it would be first seen in.

EDIT: I'm looking at the air taxi companies this thread started with, and no, they have not solved any of the relevant problems.