[FrayAdjacent]

Although my two GM vehicles have fobs, I still have a mechanical backup: a copy of the door key in my wallet (my wallet has special key pockets built in), especially if the fob battery runs down and will not operate. I bought the blanks keys on eBay with the correct security code, and had them cut for $2 each. You can buy them at the dealer for a higher cost.

All GM vehicles must keep a mechanical backup!

The problem with your solution is that it isn't one. You will be able to lock the door and unlock it, but you won't be able to start the car (afaik). The actual key fob has a passive transponder in it - most car keys have had them for a couple decades now or so. There is a detector in the key slot. No transponder, no go.