However, I've had something of a conversion on this issue after reading through and considering the implications of what is generally regarded as one of the classic fantasy RPG supplements in recent years; The Great Pendragon Campaign. TGPC takes the players on a multi-generational campaign which starts in the year 485 CE and ends in 566. During this time, the various ups and downs, plots, events, notable NPCs who wax and wane, etc. are all laid out for the game master's use. It is, by its very nature, "advancing the timeline" and it does so brilliantly.
Now, I can completely understand if fans of the setting don't like some of the specific events and themes that the later additions introduced. Having demons and devils running around the Flanaess, only to mysteriously vanish, having the western Sheldomar Valley overrun by giants and monsters, assassinating most of the members of the Circle of Eight, etc. could all be design choices that an individual DM could decide didn't fit in with his conception of how he wants to run his campaign. Absolutely understandable, and I'm not saying that's not perfectly within the DM's rights.
Historically, of course, borders change all the time, even to the extent that entire nations disappear and new ones rise. Take a look at these two maps of Europe; the first showing the borders as of 1360 CE and the next the borders as of 1400. France has taken over almost all English territory on the Continent, the Turks have taken over a goodly chunk of the Balkans, the Golden Horde is pushed out of the Caucasus, Poland and Lithuania are merged, etc.
If the face of Europe can be changed like that over the course of 40 years, I don't see any issue, from a verisimilitude point of view, of seeing changes like the breakup of the Great Kingdom and the conquest of half the Wild Coast by the Pomarj. In such a campaign, having the DM know the broad sweep of history allows him to give his campaign a sense that things are moving around the world without the direct intervention of the player characters.
Even if I, as a DM, want to change some specifics to fit what's going on in my game (for instance, if my PCs actually rescue Prince Thrommel in the Temple of Elemental Evil, that means that Furyondy and Veluna are going to be merged into a single state circa 578 or so, and that's going to have a big impact on the geopolitics of the central-west Flanaess), that doesn't invalidate the concept of having a future history timeline to provide a living breathing background against which I can run my player characters' adventures.