I wish I were a better devil's advocate, but here we go anyway:
Assumptions:
Peterson has the physical talent to be an above-average WR.
The NFL wants teams to put up points.
The NFL has created a set of rules that gives offenses advantage over defense.
Argument:
As spread offense become more popular in the NFL stockpiling offensive weapons is going to become paramount.
It'll become more about mismatches at the Y and Z guys than the top WR vs top DB.
It'll become even more about getting pressure on the QB and that's where time and focus should be spent.
Peterson's value as a #2 is great. He has the speed to separate and be a deep threat. He has the ability to make moves in small space, making him excellent for the screen game. He understands physicality. That makes him a willing blocker.
Peterson's transition should not be compared to Hester's. Hester was a man without a position coming into the NFL. Everyone knew he'd be an electric returner and it was hoped he could be 'coached' into a position. That hasn't happened. He's also a little undersized to be the #1 or #2 receiver. Peterson isn't. He's got the size and the bulk to box out defenders.
Like I said, I'm not great at contrary advocation.
But, if Toler, Jefferson, Marshall, and Adams all come back next year, then DBs will be a deep lot and have some great potential.
I think you can remove some of Peterson's defensive reps, get him one or two plays on offense at WR a game. Basic stuff, mostly decoy, but with teams doubling Larry with a safety and the other WR in the box, you could see some very cool stuff. Especially if Doucet isn't resigned and the team doesn't find a replacement in FA.