Maybe. They still have about 4 more years to work with. The last stagecoach robbery was late 1916. The last posse formed to hunt down outlaws on horseback that ended in a shootout was in Nevada 1918. The last Indian War was 1923. Rural america relied on horses as the primary mode of transportation until the late 1930s. It really depends on the story they want to tell.
I think we'll be playing as new characters but there will be cameos of younger characters from Red Dead Redemption. I'd like to corral a gang as Dutch, but I'd be up to play someone new also. The fact that we're finally getting a new Rockstar title is just awesome!
I think they're concerned that the image could indicate a connected, player-populated world which would mix elements of GTA V single-player and GTA online. Like that poster, if they don't have a clear separation between the two then I'm completely out.
Imagine how screwed up the pacing of RDR would be with a player-populated world. Think of the scene when John heads to Mexico. That sunset, that vast emptiness, that song....it was a powerful moment.
Now imagine that but CumHard420Breh rolls up, shoots you, and steals all your crap. WASTED. *respawn* minutes away.
Bleh. I too am really hoping we get a dedicated single-player experience that can be played completely offline.