(Now that car thieves have scanners, electronic devices, I don't know.) But my first thought was, what if they hijack the car, the key is in your pocket or something and you get out, then they can't switch off the car / the engine, eventually.
If it's clever enough to defeat thieves, it will be too clever for users. But in a hijack situation, the key and the owner are available to the bad guys, and they'll know what they need as well as anyone.
Personally, I think the proximity detector in the key fob was added 1.) because they could, 2.) to facilitate remote starting, and 3.) to avoid the expense of a physical key lock. Cars are ruthlessly cost-engineered, so even the cost of that will be considered by the bean counters.
My Ford has remote start, but is from the model year preceding Ford's implementation of keyless operation. It's fiddly, to say the least. Lock the doors with the main key fob, press the remote start fob twice, or some such. Then, as soon as you open the door the engine stops, and then to restart it requires the physical key. My previous Ford was actually one model year newer and had the newer keyless system. Each has its convenience.
I once rented a car that had keyless starting--my first experience with it. I parked it at the facility where I was running a workshop, and after the day-long workshop, came out to the car and discovered it was still running.
That's not as stupid to me as using touch screens instead of physical, tactile buttons. In a moving car, a touch screen seems to me a user-interaction nightmare. I'm driving in a dynamic environment--everything is moving and subject to various accelerations. Rather than running a finger across buttons on a dashboard, with just a bit of a glance to get my bearings, I now have to stare at the spot on a smooth screen and, without any way to brace my arm (which is responding to the dynamic environment differently than the dashboard), land on a colored spot that is barely as large as the tip of my fat fingers. Gloves? Nope. A bit of tremor? Too bad for you.
We all know the reason for using touch screens instead of physical buttons: They are cheaper and more flexible, being ordered by software rather than hardware. So, it's a convenience for the designer, and part of that ruthless cost engineering, but it's hostile to the user. Lexus tries to solve it with a joystick, as if that helps. I still have to stare at the screen on the dashboard to align the cursor.
(My last two Fords have had voice activation, as does my wife's Lexus. None of them can recognize my voice reliably--too deep and too close to the ambient noise spectrum.)
Rick "wouldn't be so bad if the software was even remotely competent" Denney