Felon said:
I'd say with most of those monsters you'd actually be in trouble if you were only "pushing" 20th level, and if you're not fighting legendary, ancient monsters at epic level, what's the point of being epic? What are you supposed to be doing at epic levels if not saving the cosmos? The criticism regarding the "low" CR's of cosmic threats, based on the rationale that "the gods would have destroyed them long ago" is what amuses me. It is basically saying epic-level characters shouldn't actually be out saving all of existence or doing anything of cosmic importance because if it were really important, the gods or some other high-powered muckity-muck would handle it. And for that matter, even lower-level characters shouldn't be out saving their world--don't the gods have a vested interest in protecting the place where all of their worshippers live? Who needs heroes when gods can seed the world with avatars and aspects? It's dead-end logic.
The Realms is a high power, high level world. The PC's are not the only heroes running around, and even a 20th level party aren't the biggest guns on deck, each kingdom probably has at least one near-epic or low-epic level adventuring group. Just because you're 20th level does not mean you get to start saving the entire world, defeating ancient monsters born at the dawn of time, and kill gods left and right. You don't have to save the world and rewrite the setting to have a good adventure at any level. Being high level is about more than just fighting monsters with more hit dice and adventures with higher DC's, it's about dealing with the bigger picture of the world and those who run around in it.
It is not just "the Gods would have killed them by now", it's that there are a fair number of 30th+ level NPC's who have been around for centuries or millennia (Larloch, The Symbul, Elminster, ect.) who could probably stomp on something if they were a CR 21 creature (that a 30th+ level character and some epic allies could eat for breakfast) that was going to destroy the world one day. They are supposed to be powerful enough to make the gods themselves fear them, not to be things they pummel into dirt as a warm up.
The PC's are not the stars of the setting, they are the stars of the campaign within that setting. This is not a bad thing. You don't have to be the most important person in the world to have fun, but you should be the most important people in the adventures your PCs undertake. It's a lesson I learned running Star Wars.
Just because there are Uber NPC's and monsters the PC's can't practically fight, doesn't mean that the PC's can't take part in the adventure. The Fellowship had to run from the Balrog while Gandalf fought it after all. My current Realms game is regrettably collapsing, but the climax was going to involve Larloch undertaking his master plan to deploy a huge portal network linking almost every place on Toril and much of the planes, giving free transport to all (the catch being that secretly he gets to cast spells, no-save, to anybody who uses his portals, and redirect their destinations as he pleases at will). The temptation of these portals to the public would be enough to give him enormous control over the entire world. The climax was going to feature a cameo by Elminster and a few other major NPC's as they go off to have their showdown with Larloch, while the PC's (in the circa 20th level range) have to destroy the actual matrix powering and directing the portals and it's dracolich guardian. Thus the matter of how to deal with the fact that the villain is a CR 34 Lich is dealt with by having the famous characters go off to fight him, while the PC's foil his plan and have their own challenges and dangers.
Maybe it's one reason that the Realms are widely liked by some, and loathed by others, but your PC's are not the only people running around trying to change the world. I love it, trying to pretend that high & epic level characters don't exist (or if they do stay completely out of the picture) is one reason I hate Eberron.
Even leaving out ancient eternal evils and saving the world, you've still got ancient dragons, warring nations, dangerous cults, exploring the planes, invading armies, destroying evil artifacts, and ancient ruins to explore, but Save The Entire World plots don't really work that well in that setting (personally, I also think "Save the World" plots are way overdone in fantasy and should be avoided in any setting).