Theoretically, I agree with you,If all nations would be equally able, this world would be an ideal place. But at most, this world can be a multi polar world, with a handful of countries as superpowers and the rest would still be the same as they are now. But will a multi polar world be better than the unipolar one? I seriously doubt that.
For a vast majority of third world countries, cold war era type groupings would be very difficult to cope with. And given the human tendencies, the more leaders you have the more chaotic the society will become. Multi-superpowers will play a deadly game of geopolitics, and the world would be worse than with unipolar one it is now. Imagine,if there are a handful of more US in the world, how many more Iraqs and afganistans will be there?