One could say that if deadwood were to of continued it could of suffered the way Dexter did.
and one would be wrong. By the 4th season of Dexter they were already in to the "Timmy Gets Cancer" writing, where the show stops being about developing characters and juxtaposing their situations and becomes about external forces acting on the characters. Same story with True Blood.
But Deadwood had so much development of the characters yet to be realized. Ellsworth dead, young William dead, how do Seth, Alma, and Martha continue to exist in a place where none of them have anything holding them to Deadwood except each other? The acting troup and all those relationships, past and present. Tolliver's relationship with the Pinkertons (Hearst by proxy) and his sense of loyalty to the camp. Does EB grow a spine and betray Hearst?. Fucking retard Johnny with Al, now that Al killed his Johnny's favorite whore to protect Trixie (Al's favorite whore). Trixie and Sol, is Sol the new mayor? What about Al and Silas, now that Hawkeye has returned with a rag-tag group of 18.5 hired guns? Do Joanie and Charlie grow closer, or does Joanie's relationship with Jane drive a wedge between all of them?
FFS, I could keep going, but I think you get my point.