This is a case of assuming the older design is simpler or more reliable just because it has a mechanical aspect to it. Both systems are simply an input to the BCM.
Push button start is actually simpler and has less failure points. Placing the IA fob in the cup holder accomplishes the same thing as placing a mechanical key in the ignition; the PATS transceiver reads the key. The difference is that with push button start, there is no key blade or lock cylinder to wear out, no ignition switch wiper to wear out, and no key hold in solenoid. So you’re accomplishing the same thing with less moving parts and improved reliability. And of course it’s much more convenient. This is why even some base model vehicles are now with push button start, there are less components.