Plus the D-sub connectors were standard, but anything regarding their wiring was handled by different standards. So just putting connectors where they fit wasn'

Plus the D-sub connectors were standard, but anything regarding their wiring was handled by different standards. So just putting connectors where they fit wasn't enough to be sure that it would actually work and could even fry one or both ends of the connection. With USB, as long as the both ends and the cable are built to spec, it's safe to mix and match. You'll just end up with the lowest common denominator of functionality.