I wish I had a million dollars, because if I did, I'd bet you all that next year's top two RBs are not Singletary and Wade.
I'll give you that Singletary could COMPETE for top RB duties next year. He will in no way be given an uncontested path to the job. You're likely looking at them bringing in (given no more shady or Gore) one or two quality backs, either through free agency or the draft. It has been one preseason game of the guys first season as a pro. I'm not ready to hand him the future of the backfield.
Wade is a great story, but like I mentioned above, he is in for a long, uphill battle. He's going to been to make a splash via the PS this year. Regardless of what he does, he isn't going to enter camp next year as a top option.
Given Shady's injury history, and for all the reasons you mentioned, I think they'd be insane to not carry both of them through the season. Playing time is a different story. I wouldn't be surprised to see Gore or Shady a non-injury inactive at some point throughout the year. No reason to force feed these guys the ball, but all the reason to keep as many good players around as possible.