The most interesting (and damning) part on the idea of "Sustainable PVP" is the 'P'. That is, the player. You see, PVP, in and of itself, is not a game mechanic. Sure, many of the things that go into the deed are reliant on game mechanics --locking you, activating my guns, pointing you, giving your dying corpse to my drones as a midday snack...
But mechanics don't make me undock. Mechanics don't force me to kill Charlie vs Henry. They don't make me fly in places where PVP could more easily happen. They don't force me to fit my ship. Hell, they don't even make me log on for that matter. Then, there's all the meta-game aspects of EVE where PVP interactions can possibly take place outside the actual game mechanics altogether.
So, when we ask (or demand) CCP to provide sustainable income for PVP, we are essentially asking them to provide something they have no control over. And since they don't have that much control over whether or not you log on to PVP, or become any good at it or not, they can never really make it sustainable for you, can they now.
Unless, of course, you think you should be paid for dying.
Secondly, what is and isn't sustainable is relative. Someone who only flies tech 1 fit thrashers will need much less to be 'sustained' then someone who continuously romps around in faction fit macherials. So at what level are we sustained 'enough'? When you can make 50 million an hour? Or 500? If I took a poll today of 100 people, I would get a 100 different answers.
Perhaps, they should simply reward a sum that is relative to the cost of what you kill. (What a novel idea!) But even then, they can't force you to kill anything expensive, and they can't force your enemies to fly ships of a certain cost to come die to you so that your income is sufficiently sustainable. (According to you.)
I was talking to a friend in Late Night, who admitted to me that he has made over 500,000 LP in the last few weeks simply from pvp. For someone who flies a lot of tech 1/2 frigates, this will certainly keep him in ships for a while. (And this doesn't even include all the loot he's accumulated!)
I sympathize with the desire to make more ISK. And, I wouldn't turn down any more that CCP might want to throw my way.
However, I consider my friends' story. And, I look at the rather sizable loot hoard I've accumulated as the result of my mediocre pvp skills.
And I can't help but thinking that the biggest problem for these people --the people who ardently claim that PVP is not nearly sustainable enough for them --exists somewhere between their keyboard and their chair.