If the world was full of people like you @PaulS, we would never develop anything or innovate!!! 
I've been in the technology development world for over 20 years, there is ALWAYS a way to do something! - People who say "can't" will always lag behind those who show how it CAN be done!
Here's a paper showing distance measuring using BLE... I don't need it to be 100% accurate to open the door!
For anyone else reading, here's an article describing something similar, another here with a BLE lock, and another using GeoLocation.