What about all the other people who were excluded from the movement because RMS was an ass? A common human fallacy is to overindex on the contributions of assholes, while ignoring the potential contributions of those who were excluded by their actions.
My subjective evaluation, having met RMS in person and looked at the consistent patterns in his behavior, is that he is "below replacement value" -- he has overall caused far more harm than the average person in his position would have.
In this case, though we actually could, if we wanted to, find some data. We could go around and ask a bunch of people if they wanted to get involved with the FSF but didn't, and see how many of those people were turned off by RMS. Another option could be to look at the demographics of people getting involved in the FSF before and after RMS had been outsted, and compare.
None of that is perfect, and especially the second option would only give you a correlation, not necessarily causation. But, again, our data will always be incomplete, and you just have to go with what your experience tells you will or won't happen.