Actually, it's not that hard to do proper QoS with modern switches. Just mark all packets coming from the CD player at a lower priority than the ones coming from the brake system. You can even have the devices emit tagged packets and restrict them to a subset of available priorities.
Or you could put in extra point-to-point links between each especially critical pair of devices. With IP, it's not very hard to just add an extra host route down a different pipe; it doesn't have to have the overhead that a whole new bus would.
This doesn't even get into the possibilities of using non-ethernet transport, some of which can provide more strictly managed performance guarantees.