Lots of things have tree structures -- e.g. pretty much anything with an org chart. In any case, what we're really arguing about is whether it's unsustainable

Lots of things have tree structures -- e.g. pretty much anything with an org chart. In any case, what we're really arguing about is whether it's unsustainable in some sense. If it technically qualifies as a Ponzi according to your preferred definition, but it's sustainable and everyone gets what they signed up for happily, who cares?