"just use Wi-Fi?", probably not. I don't think you'd want to create so many openings for attackers. Besides, EMI is a real issue and not something you want interfering with your critical systems too much.
To the point they are wanting to eliminate AM radio because they've gotten to the point that the uncontrollable EMI makes AM "unlistenable". So move the goal posts, and make AM radio out-of-fashion and get rid of it rather than solving the EMI issues.
2) WiFi really isn't as reliable as a cable. There are all kinds of things internal and external that could cause an outage. Why bother with that when you can just run maybe a dollar of copper to the ECU?