well, it would not be Billions of universes, it would basically be infinite number of universes popping up every second. Which while interesting, does not seem possible under any law of conservation of mass?
They're not popping up, the wave function simply branches. In Copenhagen you cut off the branches, in MWI you do not. Total energy in MWI is the weighted sum of the energy of all branches (weighted according to borns rule). If one could dismiss MWI this easily, there wouldn't be any serious proponents of it and we wouldn't have to have this discussion.