Her family can own their copies of Harry Potter forever.

But copyright restricts others from building on top of previous works. Can her family forever sue anyone into poverty for naming a wizard “Harry”? Does that even make sense in 100 years from now when most people have never heard of Harry Potter anymore?

For your building analogy, it would be more like the builders created your building there, and now you own it. It has a red roof and blue shutters. And also the government passed a law that said nobody else can own a building ever again with a red roof and blue shutters, otherwise you, the owner of the idea of those colors, can take possession of every new building like that.