Mourn said:
In 94 years time, the political map of Europe has changed dramatically, as has the political map in the Middle East. If entire countries can rise, fall, and disappear in the real world in a short amount of time, why would it be any different in the Realms, especially given the presence of magic?
Because while, for example, Thuringia, the Palatinate, etc. morphed during the Thirty Years War, Germany as a whole still existed and all of its larger towns and cities weren't blasted into the Ether.
Likewise, even though the Picts, Scots, Normans, Saxons, Romans, etc. all interacted in what is now the UK and various changes took place, London is still London. It went from a tiny little outpost to a huge city, incorporating outlying towns, etc. but it wasn't destroyed.
Basically - those places that have disappeared in our own real-world were overtaken by something else. They didn't just disappear with nothing to replace it. In most of our real world, except for inhospitable places like a desert or jungle, ancient ruins are built upon by those who follow.
It isn't that something changes in FR for a reason (war, merger, etc.), but that this is change just for change's sake.
Changes to rules should never require campaign setting information to change. If a campaign setting is required to be changed to conform to changes in game mechanics, that tells me that the mechanics weren't too sound to begin with.
Perfect example of what I am talking about: Twilight:2000. They went from v1 to v2/v2.2 with no changes to the campaign setting due to mechanics changes. They *did* change the campaign setting due to real-world events making the premise of the campaign setting become OBE, but no changes due to mechanics being changed.