I didn't say that 1500 years ago is a short timeframe. I'm saying that if you look at short timeframes like a decade or a generation or two it may look like there's not much migration going on. But stretch that a bit, and you'll see the changes. It's like a slow-moving river. Always moving, if you yourself move your viewpoint back a little.
And places become more boring if the have the same migrants as every other place, not less boring.