As is often the case, I think people are confusing FF with real-life NFL.
There is little doubt that Stewart is a highly talented RB, but in today's NFL, veteran running backs beyond the proven elite-producing guys just don't command huge bucks or top-tier draft picks. And yes, Carolina has an enviable RB stable, but that doesn't mean they are looking to deplete the depth just because they have it.
If there is a need the Panthers are trying to fill that they can't in the draft, then maybe they will move Stewart or DeAngelo if they can at an acceptable price.
Dunno, maybe as an owner I am blinding myself. But it sure seems like they had an enviable RB stable BEFORE signing Tolbert. And that signing really only makes sense to a lot of us (including some national writers) if there is another part of the plan that hasn't been revealed yet, regardless of what the coaches are currently saying.
In fact, the salary implications that you refer to are part of the curiosity of the thing. How much of the ongoing salary cap does a team really want to use on the RB position? Williams and Tolbert (and Goodman and Newton for that matter?) make a pretty nice combo by themselves from a skills standpoint, but they also represent what 10 or 11 M/year going forward? Stewart is another 2.5M this year and will likely command at least 7/8M/year as a FA next year. Is the team really planning to use 20M+ of their 120M (roughly) cap on RBs every year? I wouldn't think so, and if not, either Williams or Stewart will definitely be gone next year.
So the question is whether it would be better to move one of them this year and potentially get something in return.