Your pessimism is definitely warranted but the offense can actually be better next year if a few things happen:
1) Nassib improves or gets beaten out by one of the young QBs who simply proves to be more talented. If that happens, hopefully the upgrade in talent will outweigh the lack experience (see: Teddy Bridgewater).
2) We keep McFarlane's commitment and add another back w/ size and speed; namely Lynch, Morris, or Perkins, in that order of preference (at RB). They're all good and any one of them has a chance to come in and play over the returning backs. And I think that's very key because Marrone's "system" obviously relies heavily on the running back and as we're seeing w/ Bailey this season, not having the right back in that "carry the lumber" role really hurts this offense. PTG will likely be a part of whatever RB rotation develops but hopefully as the change-up/3rd down back and not the lead back. I'd hate to go into '12 relying on another smallish feature back. That being said, if PTG proves to be the best option, by all means, give him the rock.
3) Assuming Sales will be back, he and Lemon can push each other for that #1 WR slot and a shot at the league. If he hasn't regressed, Sales will likely be our best receiver and a legit playmaker that opposing defenses will have to account for. Hopefully a few other reliable options can emerge from the returning wideouts and the incoming frosh. Cornelius looks like a baller. Ditto for the young tight ends coming in; Thompson in particular. At 240lbs, he's already big enough to play. He looks like he can be an impact player from Day 1.
4) If Hickey doesn't get hurt, who knows, maybe he's starting by now. Hopefully he'll come back healthy and be an upgrade over Hay, with one of the other returning lineman adequately stepping in for Tiller. And if Mackey fends off any challengers to his spot, we can only hope he continues to improve along w/ the line as a unit.
While we may not be a juggernaut, we legitimately have a chance to improve at every offensive skill position next year, particularly at RB and WR, where the improvements are most needed. TE may be the exception as Provo has been our best offensive player. But we have a chance to be better everywhere else.