When a car engine fails, the default behavior is to coast to a halt -- unless driving downhill! Even so, a car has emergency brakes, gear/engine braking, a human driver, etc.
This scheme has no human in control (its "autonomous"), an externally provided destination ("connected to GPS on the users' mobile phone."), and no protection from a flying plastic bag or sheet fouling multiple propellors, turning it into a heavy unguided missile dropping onto the street below.
To the founders -- densely populated cities are the wrong place for a drone. How about delivering books or medical supplies in the Australian outback? (with a petrol engined drone)