AFAIK the issue is that most "unlimited" customers don't use gratuitous bandwidth, and most don't use more than 5GB/mo; problem is there are still some out there blowing thru dozens of GB per week, chewing up considerable limited resources.
I run about 3GB/mo. Having "unlimited" is great not so much for having endless capacity at maximum rate, as not having to worry about running out. I can deal with slowing the stream beyond an unusual amount of usage; I can't deal with a complete shutoff with a "$25 to turn it back on again" message.
The limited resource in cellular communications is actual bandwidth (measured in Hz), not anything that can be measured in GB.
I'm going to risk an automotive analogy here. Imagine if there were fuel plans for cars, where you paid a fixed monthly amount to drive up to a certain number of miles. At that point you could choose between an "unlimited" plan, where you could keep driving for the rest of the month but not exceed 10 mph, or alternately pay fixed surcharges for additional allotments of miles. The limited resource for the service provider in this case is fuel. There are people on the plan that drive Hummers and those that drive Prius's, yet despite the wide gap in their relative efficiency, the owners pay the same amount for miles. Some people pay for 1000 miles each month and never drive more than 100, other people drive 1000 in a week and face paying for more miles in a billing cycle or having to drive so slowly that their car is practically useless.
Now say I have a car that gets 15 MPG, and I trade it in for one that gets 70. My fuel service provider says, "That's great, I'll let you transfer your old mileage plan to your new car, and even raise your monthly mileage limit from 1000 to 1667 miles for no additional cost." Am I really getting a good deal here? I was paying x amount of dollars and using as much as 66.7 gallons each month, now I can go further for the same x dollars, but I'm only burning through a maximum of 23.8 gallons...
More subscribers use more spectrum. More spectrally efficient phones do not. AT&T is reducing the amount of spectrum you're getting for your dollar, Hz per Hz, when you carry over your unlimited plan to the iPhone 5 (even accounting for the wider bands used by LTE.) People need to pay attention to what they're really paying for, and in this case, it's ultimately spectrum availability, not GB. AT&T doesn't generate nearly any of the bits that they meter out every billing cycle. They license any available spectrum they can get their hands on, and then rent you very small time slices of it.
People need to be disabused of this notion that certain users who have large aggregate monthly downloads as measured in GB are somehow ruining it for everybody. The endgame here for AT&T is to increase profits for the sake of the shareholders. To grow revenues in this business you can either add more subscribers, or increase the amount the existing subscribers pay each month. If you can't easily build out your network to accommodate additional subscribers due to a lack of available spectrum in key markets, you turn to option #2. Having subscribers under contracts that cannot potentially balloon up with surcharges makes them unavailable to tap for generating increased revenue. This is the "unlimited" problem, and why these types of contracts are a headache for every carrier that isn't fighting to win new subscribers.
Rest assured, the carriers have done the mathcapped plans with a lower face value at signing generate more revenue on average during the course of a 2 year term than "more expensive" unlimited plans.