Well, I think to be a PI on a grant of any decent size what you need is a tenure or tenure track faculty position, which requires a "PhD or equivalent."
As far as I can tell, "or equivalent" means that you've made so many contributions to your field that institutions are fighting with each other to give you a tenured position anyway. Freeman Dyson, for example, never got a PhD.
Like everything in science, it's just about publications. If you somehow manage to publish a lot of significant papers, nobody is going to blink over the "PhD" qualification. That being said, the easiest way to build up a publication record is via the PhD-postdoc pathway (at least given the current system).