To me, there is nothing which seems to indicate that Shagrat and Gorbag are thousands of years old. Considering the orcish propensity for violenece, I doubt that even an immortal orcish race would have many members over a few hundred years of age.
Also, I took the reference to the Great Siege to be the siege of Barad-Dur. The orcs of Mordor and elsewhere would likely tell a lot of stories about those times which must be the stuff of their nightmares.
Getting back to the main topic, it would seem that a key ingredient in keeping a campaign very close to the real world is a strong limit on magic and monstrous creatures. This approach is typified by nemmerle and by Joshua Dyal, based on their statements.
SHARK seems to go with a world where magic and monstrous creatures are fairly commonplace. In such a world, people would likely find some way to adapt. Thus, the average NPC in such a world is higher level than in some other settings.
I think a common thread in both types of setting is that the "common man" is protected from various threats by armies and by heroes. The dynamics may be different in each type of setting, but there is that common thread. Historically, when a nation's army collapses, its populace is often subject to the attacks of that nation's enemies -- who often become the new lords of the land. (The Visigothic invasions of Italy come to mind as a good example of this type of scenario.)
I think believability depends in large part of how a world is set up and if it obeys its own internal logic. Setting the ground rules is perhaps the hardest part of creating a campaign. I have seen some high level campaigns work well, in large part, because of a good story and a good DM.
Note: As SHARK has been mentioned several times, I have left a few messages for him at his web site
SHARK EMPIRE, although the site has been fairly quiet it seems.