"the port was connected to a network card which was inserted into a slot dedicated for that very purpose"

This isn't correct. Slots on the motherboard were generic. A network card was one thing you could plug into them. Into the same slot you could also (often) put a graphics card, a sound card, a TV card, etc...

USB just took that internal, direct mother board slot and made it easily accessible externally (and changed a bunch of other stuff as well, but not the generic function of plugging in many different things).