Thanks for the explainer, the 1989 comment threw me off.

48V isn't what enables Ethernet, it does however make it more useful. 48V enables high power things to be scattered around the car - voltage loss from the front to the back of a car at 12V is an issue, and cars spend a lot of money on large wires to work around that.

High power things scattered around the car often can take advantage of Ethernet. things that cannot get a lot of power can get by on lower bandwidth data buses.

48V is a common POE standard. But Musk is again blowing a bunch of smoke.

BMW started using fiber optic cables in their cars in 2003 (with the introduction of 5 series E60) for CAN comms between modules, the reason was that in case of side crash the repair costs were super high to reconnect a baseball bat width worth of copper routed front to back. It's not new.

PoE (power over ethernet) spec specifies 48 volts, so if you want to use off-the-shelf stuff, you need to use 48 volts.

48v and the ethernet are not totally related, you could have a 48v car with the same wiring as before (just thinner wires which is the big saving for 48v). Ethernet allows even more saving on wire as you don't need point to point wiring for many specific features, you can just put a node which connects to the network and does the function you need. Old school wiring looms had a lot of point to point wiring on them. My suspicion is that future ones will be really simple as they will need only power and comms, with all the complexity offloaded to the network devices, as it were.