Is it possible to do anything using only behaviors?

#1 Technically you could make some very basic games using only behaviors, such as asteroids, pong, etc... though no they aren't mean to replace the conventional way of doing things. They are meant to enhance the conventional way of doing things :) Done right behaviors can be a coder's friend, an artist's friend, and a designer's friend.

#2 I've released a sort-of simple game and am currently working on another using only behaviors, I've even developed a way to make GUI components out of Behaviors. I do not think they are meant to replace the conventional way, I think of them as an alternative, which I prefer.