A campaign set in a future city would not be unsustainable. A Judge Dredd campaign would be, imo.
Superficially, the Judge Dredd setting has everything you need - cyborgs, mutants, supernatural menaces, alien creatures. However, there are a few things that are necessary to a Judge Dredd setting. First off, Mega City 1 is a huge place. What happens in one sector does not necessarily affect another. Thus, unless you are doing some major strangeness, it is restricted in its scope. Second, what strangeness there is, is usually stamped on hard by the Judges. They maintain an effective surveillance system and employ heavy duty firepower, with serious backup. Which brings us to the third matter: the Judges themselves. Either you are one, or you are going to come up against them.
If you are playing Judges, then your job is to maintain law and order. That's all there is to it. You will be playing high-powered paramilitary cops whose purpose is to investigate crime and enforce the law. If you are up against Judges, you need to be able to take them on or you are toast. Eventually the law will catch up on you, and it will punish you hard. In both cases, the Justice Department has the resources to do it. Panopticon surveillance, flying tanks, psychics ... If you have ever read Judge Dredd, then these are just a sample of what the Judges have access to. To coin a phrase, when the Judges turn up, the party is over.
Of course, you could set stories outside Mega City 1. Helltrekkers, Cursed Earth mutants, troggies in the Undercity. But then this isn't really a Judge Dredd game. It's just a game that uses an aspect of the Judge Dredd universe. Such games could be placed in almost any post-apocalypse setting with very little changes. Playing a game in Mega City 1 means you need the Judges and everything that implies.
Finally, I have run and played in games using all three incarnations of gaming systems optimised for Judge Dredd: the Games Workshop version, the d20 version and the Traveller version. My experience is that playing Judges is great for short bursts, as is playing perps. However, eventually you run out of cases to throw to the PCs, or the PCs make one mistake that you cannot justify the Justice Department ignoring. For long-term campaigns you need to move beyond the confines of the Mega Cities.
Of course, all this is based on my experience. Other people's mileage may vary, and I would love to hear whether anyone has actually succeeded in running a long-term campaign in Mega City 1 or its like.