That's not Blakes 7 though
Although that'd be a great way to subvert the original B7, which I think you'd have to do with the franchise.
And there are ways for the heroes to win, without winning. Actually a realistic ending could be that the rebels win, in so far as they destroy the Federation, but there's no unified free galaxy in the aftermath, instead there's chaos. Which in effect was what you had in the aftermath of the galactic war in Series C. I mean much as I do actually like Series D (mostly) The ending of Series C might be the best the rebels could hope for. The Federation has been decimated, and with Servalan dead I could see the remains of the Federation fragmenting. Liberator is destroyed and whilst our heroes are alive, they're potentially marooned on a dangerous planet. On the one hand it doesn't have the downer ending of Series D, but conversely it isn't the end of Return of the Jedi either