If anything, they would build a car that is accessible to everyone, and have better iDevice integration to push iDevice sales. Makes no sense to do it the other way around.
1. PC - Initially made for one particular store out of garage. It benefitted them to focus on just that store, because it gave them a feel of exclusive store without having one. Customers would come in, and buy there, because there was no price comparison possible with other stores. With price comparison came product comparison, and all PC companies as small as Apple lost business because of that. This was also accidentally done as Apple got a sizeable order from one single store, and they didn't have proper bandwidth to deliver even that.
2. Music - Initial iPod was Mac only. Steve mentions over and over in documentaries and books that it was the only reason he could convince music companies to let him sell music for $0.99 on iTunes. Too small a product, they thought, for tiny Mac-only player, it would be a good publicity thing. Had iTunes was a global platform from day one, it would be selling you full albums, with a complimentary CD delivered to door. Shipping and handling additional.
3.iPhone - AT&T only. And on this one people cried. But it helped Apple with becoming the OTT king, because they could always say, there is not propertization of telephony here, we only work with one carrier.
Self-driving cars will have a battle with a lot of authorities, in a lot of countries. Nobody knows how many turtles exist in that cave. So it helps starting small.