To be honest I am not sure the realm should have outlived the dragons. They were the only thing allowing a medieval king hold sway over such a huge area. The only comparison from actual history I can think of are the mongols but
A)That soon fell apart
and
B) They had a hell of a more more organization and infrastructure than the Iron Throne ever did
One of the stranger things to me was the fact that Westeros is even as culturally homogeneous as it is. Realistically I think people should be speaking distinct but related languages in places like Dorne and the Vale. I mean I kind of played fast and loose with related languages in Essos and Valyria was only 400 years ago. The Andals who are the last continent wide linguistic influence are millennia old. All well and good to say that Old Tongue +Andalic = Common Tongue but if we look at the real world Latin + Germanic = All the Romance Languages. They might have been mutually intelligible at first, but thousands of years later like no just no. Westeros is way too homogeneous.
I mean, Spain managed to keep a colonial empire around for quite a while on strict feudalism and pretty much all former Spanish colonies kept limping along with quasi-feudal system to this very day. Mexico still is a feudal state wearing a republican mask, while strongly Mexican areas in the US were acting like feudal fiefs of local potentates until well into the 1970s.
Running a realm the size of Westeros on feudalism is possible, but Martin gives so little in terms of political, cultural and institutional background that there is no real way to say what exactly should keep a realm together.
It barely makes sense for regions like the Vale, the Westerlands or the North to have been unified. The Riverlands at least had a sensible amount of instability. Oldtown and the Mandervale should have been distinct kingdoms.
I could go on for a while...
As for the languages, compare the old Latin language of 400 AD to the local tongues of the successor states in Iberia, France, Italy and so on by 800 AD. Aside from the heavy Germanic influence in many areas, even relatively pure languages like Romanian had heavy drift by then. Even church Latin, which is by far the purest form of Latin to survive the fall of the Western Roman Empire, had marked drift by then.
It kinda makes sense for Essos to retain mutually intelligible versions of Valyrian due to a mix of cultural preservation through worship of the Valyrian Freehold and the fact that there were no signifcant barbarian conquest of former colonies, but still. Braavos having a version of Valyrian at all is supremely odd. By all means, Braavos should have it's own language that is at most loosely related to Valyrian.
As for Westeros, there should be dozens of languages. Heck, there should be at least three variants of the original First Men language surviving to this day in the Mountain Clans, the rural North and Beyond the Wall, and given the massive drift over the ages, they should be mutually unintelligible.
Why is Dorne not speaking a hodgepodge of Andal and Rhoynar? And where is the weird language isolate that is the equivalent of Basque?