Epic Rules R00ls!
Ahem. Sorry. I just couldn't resist a clumsy rant.
But, to show you that I can pull off more then a simple leet-speaked commentary, here goes.
First, it is suggested that, with such powerful creatures, how do normal beings exist? Well, really, that's an ecosystem problem with 3e. If you look at a Colossal dragon, who weighs about 250,000 pounds, apparently, this dragon eats a *lot*. And has had to eat quite a lot, to get such a Big Healthy Dragon. Given that, how could it possibly exist, since it could wipe a state of cattle out?
This is where You, the DM, come in. Obviously, a dragon could not eat all the cattle it needs, all the time, to survive. Thus, we use common sense, or a little creativity, whichever. Let's then say that this dragon has a slow metabolism. Like alligators, who sit in the sun while digesting their food, it could take a while for a dragon to get everything. Another likely thing is, dragons could hibernate. They eat quite a lot, at one point, then lay down... and *sleep* for a decade or so. So, now, we have less cattle dieing to feed Mr. Dragon.
In Issue #296 of Dragon, the Thorcisaid is shown, an epic monster. It's a roach like parasite, that basicly eats by draining levels and abilities. Since this is basicly drinking a host's spirit, soul, or 'energy', there's a lot of leeway on how this creature's metabolism works; how often it needs to eat, and how many people are needed to fill it. Obviously, a 15th level character is going to fill this bad boy better then a 1st level one. So, who says it has to eat an entire city? It could only need One person, or 4, or 8 a year, to fill it. Besides, it could also have a very low birth rate, since the most they're ever found in is 4-7. So, no, these wouldn't destroy a city, if a DM tooled them to.
Also, think about this: All these creatures, if they were all massed in the world, possibly could destroy the population, except for 2 factors. 1) They likely could eat other monsters. I could see three or four of these suckers taking down a dragon. 2) Other Planes. Yes. A LOT of the higher CR monsters ARE outsiders. Thus, they're likely Not Here the majority of the time. And, could very well be terrorising some other plane of existance. So, yes, they could exist.
Infact, the whole campaign could take place somewhere *other* then the material plane. I the PCs can access magic to just zip plane to plane, then likely, they're not going to stay on their birth dimension.