I think Cobb is the heir apparent to Jennings, but don't think Jennings will walk when he is a FA. GB does an excellent job of getting WRs that fits their system, and bring them along slowly. If Cobb shows something in the next two years, GB will lock him up before he can test FA.
I don't expect much from Cobb this year unless they let Driver go and he plays the slot in 3 WR sets (read somewhere that Nelson may get some slot work, too), but I see his production increasing over time and should startable for FF in 2013.
What will be interesting to follow is the target distribution for all the "mouths to fee". Because GB is so effecient, there are not as many attempts or completions as some other high octane offenses. Rodgers was on pace to throw "only" 535 passes last year. The pecking order is pretty apparent, but the way he spreads the ball around, I have a tough time seeing 4 viable FF plays for WR/TE.
Jennings got about 8 targets a game, and I don't think that will change. Nelson and Finley each got about 6+/game, sometimes game planning for Finley more some weeks. I expect Nelson to get a slight bump up in targets and Finley to remain the same (disappearing for home games last year should be a concern... and Jordy was the opposite. Weird.). So that is 21 targets if I assume an 8/7/6 breakdown.
If Driver should leave, where do his 59 targets go? Maybe they run more 2 TE sets with Finley and Williams, but in the past they have never really done that. Cobb moves to the slot on 3 WR sets and Jones is the 4th WR? Or maybe they alternate packages and get roughly the same targets. I think Cobb is a much better prospect than Jones and is more versatile. But does that translate to 5 targets a game? I really doubt it. And as someone who has owned Henderson and Meachem, I can't trust rolling a player out there that gets less than 5 targets a game.... so I am not counting on Cobb in any of my Dynasty leagues.