>The only problem is that at a certain point, you weaken the donor hives.
If you take frames of eggs and very young brood without the adhering bees and swap it with a drawn comb from the other hive, it makes no difference at all. They have very little invested in eggs. The queen can lay thousands a day. They have a lot invested in capped brood as they had to gather a frame worth of pollen, a frame worth of honey and a frame worth of water to raise them to where they are capped.