As somebody who was an early NeRD (NeXT Registered Developer) and helped start one of the NeXT User Groups, I don't think I would call NeXT a successful company. It created some great technology for its day, but it was an ongoing commercial failure. Nobody would buy the hardware, so they killed that early. By the end they were killing off the OS and going in the direction of being a set of Windows development tools.
It was only that Apple wanted Jobs that saved them, that and Steve's enormous ego, which demanded he bring NeXT along. At the time, Apple could have bought Be and I expect it would have gone as well or better. Remember, it took them 3 years to get a new OS out the door, which is a long time in market-land. And really, the NeXT stuff didn't make a giant difference; it wasn't until the iPod that Apple really took off.
It's worth noting that Steve Jobs's major skill, aside from yelling at people until they designed something that he liked, was marketing. Jobs was incredibly good at selling himself. When weighing his "genius", I think it's worth looking at the extent to which his public image has been shaped by his own skill in self-promotion, and the fact that his company made something that a lot of people directly use and love.