I'm guessing that Diana did not have the skills to be a soloist early on. Yes, she was blessed with a singing voice, but it was at times on the "strange" side, which probably lent itself well to a group scenario, but might have been less attractive on its own. With singing groups being the way in for a lot of young people at the time, it's likely she may not have pursued a career as a singer on her own. And without those valuable years from 1959-1964 where she is honing her skills as a vocalist within the Supremes, she may never have developed into the quality talent that she ultimately became.

Now, had Flo and Mary grown so disillusioned by the lack of success for the group that they both dropped out prior to "Where Did Our Love Go", I think Diana would have forged on as a solo. And while she still would not have yet had all the things that eventually made her a worldwide sensation [[as a solo), I think she had enough of the "it" factor to continue as a solo from "Where" on forward.