The Best of All Possible Worlds?

Leibniz, a German philosopher, supposed that we live in the best of all possible worlds?
How has he come to this thought, and why this claim is so really strange I'll try to demonstrate it below.

1. God is omnipotent, omniscient, and omnibenevolent;
2. God created the existing world;
3. God could have created a different world or none at all (i.e., there are other possible worlds);
4. Because God is omnipotent and omniscient, he knew which possible world was the best and was able to create it, and, because he is omnibenevolent, he chose to create that world;
5. Therefore, the existing world, the one that God created, is the best of all possible worlds.

[the material was taken form Britannica.com]

If Leibniz the fifth conclusion was true, than it would lead us to the very fair Christian decision:

6. If the existing world is the best of all possible worlds, then the promised paradise is not the best of all possible worlds.

Indeed, why to leave Earth, if it is the best place ever?? Leibniz's conclusion looks like truly horrible if we add to his set of arguments this one:

7. If the paradise is worse than the Earth, then there's no reason to leave Earth trying to get to the paradise.

So, why try? Although, we can get not a bad lesson refuting the Leibniz proof: 

8. The existing world in not the best of all possible worlds.