To be fair, the computer in a USB MIDI setup where it's just connecting a physical controller to a physical synth is basically just forwarding MIDI messages ver

To be fair, the computer in a USB MIDI setup where it's just connecting a physical controller to a physical synth is basically just forwarding MIDI messages verbatim from one port to another. You can buy MIDI host devices that really don't do anything at all except allow you to connect two USB devices and have MIDI work. (They tend to be expensive, and they're kind of annoying to have to deal with.) This isn't a proprietary lock-in thing, it's just an unfortunate limitation of USB. Though I suppose a hardware manufacturer could release a product that only works with some special app if they wanted to go that route. (Some devices have settings that can only be accessed via a computer app.)