I was just about to suggest the same thing, Bild. A USB-to-ethernet adapter would be an ideal compromise for those that prefer to be wired.
That said, if they're already incorporating networking technology, the cost of a physical ethernet port would be negligible. They're incorporating USB ports for no particular reason, right? Why would an ethernet port make less sense?
If they don't include an ethernet port, it's likely a design choice, not a cost one. If nothing else, having the option to use an adapter like this over the USB port would be a good alternative. It wouldn't be a hard thing for Nintendo to do.