A research professor typically graduates dozens of PhD students. Perhaps there was a post-war bootstrapping period where every one of those students got a tenured position somewhere, and in turn also trained dozens of PhD students; but it's pretty obvious it's not realistic to expect this to continue indefinitely. We're way past saturation right now. Certainly very few are going to get their own tenured positions, and as for the rest, it depends on the winds of funding availability in industry.