There were some official trailers made a few years ago for a "digital release" that included this line, and to be honest it didn't seem to me like one that "had" to be used to promote the whole saga. If I'm remembering it right the release preceded Rogue One, which made the other spy caper of the saga into a bloodbath... (That the original Star Wars didn't include a similar commemoration of loss is another reason why Rogue One wound up disconnecting from "the six-movie saga" for me, but I keep telling myself dwelling on that sort of thing isn't healthy.)

"Bothans" wound up being played up in Timothy Zahn's first three novels and the "Expanded Universe" that followed, if mostly as an unpleasant politician making hay off the sacrifice of his people. I know they were a sort of furry alien, but have to admit to having wound up with little more interest in that earlier extension on the movies either.

Anyway, your comments do add more weight to this line than the trailers I mentioned did for me, and I also thought your comment about "a revolving door of leadership in the Rebellion" was an interesting one, even if "a young Mon Mothma" was cast in Revenge of the Sith and had some lines in the deleted "dawn of the Rebellion" subplot. I suppose I'd thought before she was part of a different Rebel group than the one in the first two movies, one that linked up with the heroes in this one.