This is an interesting point. I think the difference here is that no car could be engineered to withstand an artillery shell, whereas we can imagine an iPhone

This is an interesting point. I think the difference here is that no car could be engineered to withstand an artillery shell, whereas we can imagine an iPhone not susceptable to this particular vulnerability (it already exists). Perhaps one argument is that the space of _potential_ vulnerabilities in something as complex as an iPhone is so huge that it just isn't feasible to create one that can withstand all network-based attacks (in which the attacker hasn't obtained user consent, Apple's signing keys, etc)? However I'm not sure if I buy that argument, or not...