If one bitcoin could buy all the wealth in the world, it would make the other 20999999 worthless. It's not logically possible.
Technically it is possible. If the remaining coins are lost (eg sent to 1BitcoinEaterAddressDontSendf59kuE by mistake), and all other world currencies become obsolete, then that last remaining bitcoin would represent all the wealth in the world.
No, only the money value in the world, not the value of useful things and services.
Edit: Then we have credit, which always will extend the money to some degree. (edit2: , making that last bitcoin's value less.)