On a completely random note, I think the amount of time to do this attack, even with the current setup, would make it nonrealistic. Someone above listed the steps to break into a Tesla using this vulnerability (how accurate they were, I don't really know - or care for that matter). There's one big factor that is being overlooked, however. With relatively few Tesla cars on the road right now (I don't know the exact numbers at the moment, but compared to all other cars on the road, I think we can agree that "relatively few" is a safe estimate), this particular attack isn't one that could be done with the "normal" way that I imagine stealing a car goes:
"Hey that's a nice car...lets steal it!"
For this attack to work, it would have to be done one of two ways:
1. Break into "random" Tesla accounts until you found one in your area
2. Exploit this attack to steal the car
1. Find a Tesla parked somewhere.
2. Somehow figure out that car's account
3. Break into that account
4. Use exploit to steal car
Basically, the time it takes to break into one Tesla account is irrelevant. The goal is to break into the RIGHT Tesla account, which I imagine, unless you already knew a lot about the owner of a particular car, would take a lot longer than this 69 year number being thrown around for breaking into a single Tesla account by brute force.