It's as unrealistic as having all the roller coasters, rides, and themes from PC1 on PC2, and that's exactly what's happening...
I can understand that not everything might be available from launch, but I think they can keep adding free content (which we've already paid for) with updates.
What good is it (in terms of diversity) to have flying birds and aquariums if I'm going to lose the land animals I already have? That way, I'll never be able to create a diverse zoo, let's say in South America. I'm going to have toucans and arapaimas, but I'm going to lose the ocelot or bush dog I just paid for...