You may have a point
The iMac used it as a standard and got rid of all legacy ports. Meanwhile in the PC market USB ports were already standard, but legacy ports were included on most PC systems. Why make a USB device when 99% of the PC market still has serial and parallel ports? I think that the iMac was a factor, but not the only factor in getting companies to make USB devices.
"In order to completely solve a problem, you must make sure that the root of the problem is completely removed! If you leave the root, the problem will come back later to get you." - Norman King