The companies failed, but did the founders end up worse off than if they had been random engineers at Microsoft?
So it wasn't wasted time in that respect, and they'll be bringing in unique and different skills that they probably wouldn't have developed had they been at MS the whole time.
But...there's things like the lower pay they probably had while doing their business, and the intensity of running a failing business is very very hard mentally and physically.
Near the end, one of the guys shut down...hard...now that he's moved on he's recovering. But I think that kind of experience trails along with you for a long while.