I'm aware of the points you raised in the first part which is why I mentioned that I wouldn't have a problem with two siblings who met as adults having a sexual relationship. In the case of childhood friends, there's no inherent power dynamic so it's fine.

But it’s difficult to come up with specific rules on exactly how much of a power difference is acceptable, and there are plenty of troubling relationships that are universally legal, e.g. all the Donald and Melania-style relationships between rich old businessmen and much younger and poorer women.

Agreed, but just because it's difficult doesn't mean we shouldn't try. The difference in power between parent and child is very clear and severe and is obviously across the line. And obviously there needs to be a line somewhere, or else we'd have to say that prisoner/guard and slave/master (actual, not the kinky stuff) relationships are also fine.