[elihu]

There are still many reasons to prefer MIDI cables over USB. The main one is that you can plug pretty much any MIDI device into any other and expect it to just work. With USB, you almost always at least need a computer or MIDI host device to act as an intermediary. MIDI cables can also be a lot longer than USB and (if the receiving device implements the spec properly) the devices are optically isolated.

There are a lot of potential technologies that could improve upon the antiquated MIDI cable standard, but USB is actually worse in a lot of ways.

CAN bus might be a good option; it's fast enough, and is already available on a lot of cheap microcontrollers. I haven't worked with it, though; maybe there's a shortcoming I'm not aware of.