Cobb is not a #1 type receiver. Cobb is a unique talent that can do a combination of things that makes the old distinctions among "X", "Y", and slot inappropriate. In a way, it's like Sir Charles on the defense. Last year he was called a corner, this year he's called a safety, but in reality he fits neither position. He's sort of like a next-generation rover back. Similarly Cobb is part slot receiver, part Darren Sproles, and part something else. A pre-Otto Graham quarterback and a bit of Philip Epps and John Jefferson.
Putting him as the #1 (is Jennings the X or the Y, I can never remember) would be a waste of his talent.
Personally, I think the only WR on the roster with the potential for filling Jennings' role is Jones. I just don't think Nelson has the range of skills/endowment/whatever that you want. For some reason, I keep thinking of Alvin Harper. It isn't that I think they have the same skill level (Nelson is better) or are even the same type of receiver. But Harper was a great #2. But he was an awful #1. I don't think Nelson would be an awful #2. He'd be above average. But he'll never be Greg Jennings, much less Sterling Sharpe or James Lofton. He's missing something.
Jones has more to offer. At least potentially. I'm not sure he's got what it takes either, though. Being #1 is not just being able to make the tough catch (which he's always been good at), being strong (which he has always been), and be dependable (which he hasn't always been. There's something else. Some "it". I don't think there's anyone on the roster right now, other than Jennings, who has that it.
Maybe Troy's right, maybe we don't need a classic #1 anymore. But if we do, until Jennings gets back to health, we don't have one. And if Jennings leaves after this season, which IMO looks more likely every week, and which WILL happen if the Packers win another title this year, they won't have one from the remaining current roster next year.
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)