[PinguTS]

This adds complexity and with complexity there comes a price tag. That would make the key fob more expansive. It also adds higher power requirements this then comes with new requirements for the battery.

[mppm]

Re price tag: you can buy a smartphone for 100$. Surely it is possible to mass produce cheap key fobs with send/receive capability and a tiny crypto module.

Re power: Key fobs already do some form of crypto and broadcast. Adding reception capabilities ought not to be that power hungry.

[Iolaum]

Even Better, they can use a smartphone app. We already have a battery-powered device that can emit radio signals in various frequencies!