Hylian Dan alot of what you say is good about a HBC, but in regards to your points about the appeal and risk of a HBC having no human characters...well, there has been in the past quite a few theatrical films which were succesful without invivling human characters at all, for example -
a bugs life, the lion king, wall-e, the land before time, happy feet, ice age, dinosaur just to name a few. all of these movies, with the exception of the lion king and the land before time, are all CGI - which as you said, a HBC would be. And each of these movies featured no human characters at all (maybe ice-age 2 had a little baby caveman human girl or something, but thats ice age 2 i think, ice age 1 itself was successful as well).
So, all I'm saying is that its not unheard of, not impossible, for a movie to start of with non-human characters and for the rest of its duration to have no human characters at all, to be successful.