What motivation does he have right now to do all the things he does? So he got about $100M from the sale of Paypal a while back. He could have easily coasted on that and led an easy life and never have to work a day any more. Why didn't he?
(I'm not glorifying him here by saying that he would allocate all the resources in the world fairly, neither that he should or should not be given that power, in our little thought experiment here.)
Whatever his motivations may be, we couldn't possibly allow any one person to decide over our finite resources. So, how to stop that from happening? Should there be a some kind of cap? Or maybe take some progressive amount from everyone based on some rules? And so forth.
Yes, what motivates somebody like musk after they earned 100m? Money? Or doing something relevant, challenging, satisfying? Real innovators aren't homo economicus, they don't do everything just to maximise future earnings...