Identities aren't limited as anyone can create a comet, an identity without a parent (self-signed identity). There is a hierarchy of limited identities which serve as a routing network to propagate software updates and network packets. Since in reality you need to choose some computer to receive software updates from and send network packets to. Comets could communicate without the galaxy/star/planet hierarchy but they would have to choose some other way to route packets (DHT or something).
Urbit can function without Ethereum. AFAIK galaxies (super nodes/identities) participate in Ethereum as a consensus mechanism voluntarily. Nothing is stopping them from changing that.