A rookie wage scale might fall under the "be careful what you wish for" category.
The harsh reality of the NFL is that it chews up guys and spits them out. No one gets paid for what they've done, only what they're expected to do in the future. And with non-guaranteed contracts, the gravy train is over as soon as the team doesn't feel like paying it any more, for whatever reason. That's why every veteran gets as much money up front as he can.
So, if they're not going to pay top rookies because they're unproven and they're also not going to pay them like they're expected to be stars, even though their draft position suggests that's what they believe, everyone knows what's coming after that: holdouts. These guys aren't going to wait three or four years to get paid, the nature of the sport being what it is. So they're going to demand new contracts the instant they become "proven." Or else they'll simply refuse to sign long-term contracts in the first place.
I also think it's a solution in search of a real problem; only the top half of the first round is making excessive money and most teams aren't pressed against the cap anyway, which suggests the savings will be pocketed by ownership more than redistributed to veteran players.