The data rate over USB2 after accounting for overheads is a lot faster than 100Mbit, so it’s not entirely pointless - you would be able to serve cache files fas

The data rate over USB2 after accounting for overheads is a lot faster than 100Mbit, so it’s not entirely pointless - you would be able to serve cache files faster for instance. But as soon as you have anything else hitting that bus, your real world bandwidth is going to be < 100MBit. Since 1G is more power hungry than 100MBit interfaces as well IIRC you can understand why the Pi people decided to stick with 100MBit.