The reason salary caps work in real life are the same things that make salary caps untenable in an MMO like this.
1. The most important reason is that Real Life is a team's market. Players aren't technically forced to sign with the team that drafts them, but I don't think there has ever been a player that didn't. The draft allows teams to actually go out and say, "I'm going to hire this player, he's going to really help this team go to the next level". That does not exist in GLB. There is no mechanism for getting players to play on a team they don't want to play for. In RL, you can trade for or draft a player and he'll play for you, because otherwise he makes no money that entire year. In GLB, there's nothing you can do if a player just doesn't want to play for your team. Technically you can trade for him, but, yeah, I don't remember the last time I saw an actual player trade, especially without the player's direct approval.
And you know what? That isn't a bad thing. GLB is billed as an MMO. It's right up there on the top banner -- "American Football MMORPG". Part of being an MMO is playing with your friends in a guild/clan/house/team. If there were mechanics to force people to play on teams they didn't want to play for, they would just quit. Why shouldn't they? Their livelihood isn't dependent on it. This is a game that they're supposed to play for fun. It's generally not fun being on a team that you can't stand. Imagine if you were playing World of Warcraft, but you couldn't decide which server or faction or guild your character was on. It would not be even close to the most popular MMO on the planet in that situation.
2. Another reason that makes salary caps work in real life that doesn't translate to GLB is that players actually have stuff to spend money on. Players actually want more money because they can spend it on luxury yachts, fancy cars, home entertainment systems that rival theater complexes, and other such things. Players almost invariably try to get as much money out of the team as they can (occasionally you'll get players who voluntarily short their own salary to help a team out, but they're very rare compared to the total number of contracted players). That doesn't exist in GLB. Players have absolutely nothing at all to spend money on. Literally. With the training and equipment system change, player money is just a point counter. It's useless. That means players have zero incentive to actually negotiate a higher salary, which means there's no reason for owners to pay them anything but minimum wage, which means the salary cap is doomed from the start.
Now, you can try to force the issue, like the revised proposal does by dictating the salary amount, but that's just a very heavy-handed bandaid that doesn't address the root problem. Fix the disease, not the symptom, if you're going to institute a salary cap. EDIT: Also, just throwing in meaningless "bling" items won't do a thing to fix this issue, because meaningless items by definition have no meaning. There's still no incentive to make more money so you can buy something that is completely useless.
3. The third reason that salary caps work IRL but not in GLB is that teams are really frakking expensive. There are several teams in the NFL that do not make a profit (I make this statement because I know for a fact that the Jaguars aren't making a profit currently, and despite the media attention they aren't the worst-off team as far as selling tickets is concerned). I don't think there's any teams that have been owned for more than one season that don't make a profit in GLB. I don't think there are any that even come close to running a deficit. This is actually the entire reason that the salary cap was implemented in real life in the first place; to promote parity between teams that had such a broad range of revenue streams.
Now of course someone's going to say, "But this change is done to introduce parity between the teams too!". To that I say, "read the three points over again". Money doesn't matter in the game, friends matter. If you make it so people can't play with their friends, they aren't going to say, "Oh well, I can't play with any of my friends, I'll just keep playing even though I signed up to play with my friends". They're going to say, "This sucks; I quit". This is established MMO fact. Every single major MMO that has introduced hard population caps on servers or otherwise limited the ability of friends to play together has backpedaled on that almost instantly. Why? Because people play MMOs to play with their friends. MMOs are inherently social games.
Take away the social and you take away the game.