And incapable of most common USB use-cases even at the time.
Also, "simple" has to mean something else in your world because I distinctly remember the onus of having to figure out the right baudrate, stopbit and parity bit configuration. Not to mention devices which used TTL level RS-232 and wouldn't work with other devices DESPITE having the same connector and standard.
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.
It can be very cheap, very fast, very powerful and still be compatible with most* devices.
Any other standard trying to fix one of the aspects will drop the ball on the others - e.g. see Apple's attempts at Thunderbolt 2.