That severely depends on legal and social system you operate it. It is not universal truth, that was artifact some concrete historical period. And the loss is frequently result of large war, revolution, violent displacement, that sort of thing.

That's an awfully long runway.

Mate, I’m visiting Europe right now. The tail of generational wealth is so long here you can even see it. We can certainly see a few examples of family wealth being squandered. And it makes a great narrative about karma and balance. But I’ve met even more families that while not individually wealthy will never, ever be poor in the foreseeable future for the next 5 generations at least.

Is that summing the wealth of every single person in a (usually growing) group of inheritors, or averaging it?

If 1 guy with $16 billion turns into 16 grandkids with (inflation adjusted) $1 billion endowments each, it sounds to me like the family fortune was preserved extremely well, the family is simply bigger.