Well, it'll change it, but we'll see how much, in truth. The holdings that are already full of building slots will have a huge head start, and it looks like it costs nearly as much to create a new holding slot as a new holding. Depending on how long it takes for those events to fire, it might be quite the uphill battle to make a bad county worthwhile.
Right but I'm talking macro, looking at your whole run long term. I'm saying this will drastically affect the question I always ask myself of do you want this to be your capital for the entire play through.
I think in my games there is the notion that you started here I tend to stay in that duchy, or near there. But there are other games where you get to the top title in the area and you start to re-evaluate where your capital is.
It's like
@Kozmkz said, instead of say gravitating toward Brugge for it's six holdings you're going to be tempted to maybe not. This will make deciding to move your capital duchy much more significant.
And maybe there is a cap that they simply haven't mentioned, yet maybe we won't be able to go up to 6 holdings with every province. Who knows.
But honestly the amount of time it takes is irrelevant, even if it takes 400 - 500 years to fire, + keeping up prosperity levels, even with both of these the ability to build holdings will change how you evaluate your capital / potential capital, for the better I think.
Flanders and Thrace, and Rome will still be top choices, but now you will have the option of maybe staying here and actually building your "family land" to potentially be as good, even if it takes centuries.
Edit and this whole prosperity system will help what a lot of players think is too slow of a "peace" game.