So you agree that the cause of civil rights started out as the underdog, and only gained power gradually with time until it had enough power to challenge the established law and change it. How does this differ from the standard Progressive narrative? (other than, perhaps, insisting pedantically that once the cause is strong enough to win, it shouldn't be called the underdog any more?)
Re your last paragraph, it is true that if a winner writing history says "we were the underdog, and despite that we won quickly and decisively", this should be suspect as unlikely and a self-flattering image. But if the winner's history goes "we were the underdog, had many defeats and setbacks but gradually rallied people to our cause until we started winning, and we hope to win more in the future as more and more people come to side with us" (which is closer to the standard Progressive narrative on civil rights, women's rights, gay rights, etc) why is this especially suspect? (You might suspect the cause to be less pure or the reasons why it gathered support less related to people seeing its justice, than the supporters believe, but this is different from questioning the underdog to overdog progression story).
Progressivism was already utterly dominant in the 1960s. It was utterly dominant in the 1900s. What changed was how important it thought "civil rights" where. This did not happen due to popular sentiment but changing moral fashion among intellectual elites in general. Not only did popular sentiment not change much because of activism, neither did intellectual moral fashion, it was changed as a side effect of where Ivy League opinions where a few decades before.
Now sure those opinions might have shifted because of activism, but that was a different generation of activists than the ones that where picked by the media and education industry as symbols for their new prescription for society.
http://unqualified-reservations.blogspot.com/2013/01/noam-chomsky-killed-aaron-swartz.html
Summary: Moldbug on the Aaron Schwartz affair. Power is a very real thing with real consequences for activists, yet many people don't understand the nature of power in modern times. People like Noam Chomsky get great fame doing bad epistomology about who has power, and as a result do great harm to idealistic nerds who don't read between the lines to selectively target their attacks at weak institutions (Exxon, Pentagon) instead of strong ones (State, academica incl. MIT).
Here he returns to a theme that is one of his real contributions to blogospheric political thought: that victory in political competitions provides Bayesian information about who has power and who doesn't. If your worldview has the underdog somehow systematically beating the overdog, your epistemology is simply wrong - in the same way, and to the same extent, as a geocentrist who has to keep adding epicycles to account for anomalous observations.
This means that activists like King, Schwartz, and Assange are only effective in bullying the weak, not standing up to the strong (despite conventional narratives that misassign strengths to institutions). When such activists stop following the script, and naively use the same tactics to attack strong institutions, reality reasserts itself quite forcefully: