Slim profit margins and a saturated market were both comments people made regarding the phone industry before Apple entered it.
Those are only a problem if you plan on just entering the market and not being a disruptive player, if you're just planning on being a me too player.
If Apple moves quickly enough on making an autonomous vehicle, that could be a serious disruption to the market. I'd be willing to pay over a year's salary for a car if it ran reliably for over a decade and was fully autonomous - that gives Apple plenty of room for profit margin, I think. If Apple enters with a product like that before anyone else has it, the competition will immediately fall apart just like RIM/Blackberry/Windows Phone did.
But Apple will have to move quickly, because Tesla is already barreling down that path. Their already available cars require little maintenance and have a lot of automation. And, last I heard, they said they would have a car available for ~$40K by the end of next year... if that car comes with comparable automation features, they could beat Apple to the punch.