There is, of course, the practical reason of not switching to dynamic borders, that being how much of a complete and total pain in the ass it'd inevitably be to make a save game converter. Other than that, you'd need a whole new model for the game's economic system, since in Paradox games economy and technology rely heavily on the province system. Now, with that, look at it from Paradox's perspective. They'd have to decide to take a tried and true system(even if it isn't perfect, it still works) and scrap it, then design a completely new and different system that A) May or may not work, B) May or may not be better, and C) Won't necessarily please their fans. If you contrast those variables against what actually would be achieved, is it really worth the risk? Though, I do enjoy the idea of something more along the lines of the Civ map, or even the RTW map if you could actually take over a salt mine instead of having to assault a city for it. Being able to make tactical decisions like that does intrigue me, because if you attack an enemy's key financial interests you can possibly draw them out of their strongholds, instead of having to fight siege battles all of the time(The computer is very reluctant to fight if it doesn't have to, ironically, and I don't always have the patience to wait for them to starve). Though, even with something like that, it still might be too much of a pain in the ass for people who aren't as interested in war.