BLE beacons do a crap job of detecting distance and battery life is just OK (a few months typically). BLE can obviously be used to differentiate between in-range and out-of-range which is useful for some applications but you can't really use it to measure distance more granularly.

You just need BLE for proximity to wake up a smarter portion of the fob for an instant to do the time-of-flight check

I'm just trying to think of any reason the keyfobs NEED to use low frequencies, and all I can come up with is cost