I think the insight here is that garbage is garbage and the generational hypothesis holds for DB trees as well. So if we track and treat young nodes specially,

I think the insight here is that garbage is garbage and the generational hypothesis holds for DB trees as well. So if we track and treat young nodes specially, we can then promote a subset of them into the permanent tree and dispose of the rest. Per the hypothesis, nodes in the permanent tree will also die, but at a lower rate.