What happens if you run out of battery? Or worse, what if you lose your phone or it is stolen?
Isn't this creating a security hole to fix a problem that really doesn't exist?
(Yes, I understand that it's possible to lose keys too, but typically we don't take out our keys 500 times per day. On my most recent vehicles, and I suspect this is true for all vehicles now, the key can stay in your pocket. Lexus used to have a credit-card shaped key that would fit into one of the card slots in a wallet, so it didn't even take up extra pocket space.)
The Tesla Model S has a key that uses proximity, but you can also open and control it with just app - all except driving it away. Other than the size of the key fob (for a tech company, it surprises me how big the thing is), that strikes me as the most reasonable compromise.