If there's no afterlife then there is no point in moderation. One can afford to be charitible if they believe that their reward after death will be a gold palace in heaven where angels give them mint juleps. I personally think Mother Theresa is probably living in something now that makes Buckingham look like a tar paper shack.
However, if this is all that there is, then you need to get as much as you can fast as you can, basically, if your hypothetical is ever proved true, all you will do is create a lot Neros and Caligulas, the murder rate would go up and at some point societal advance would just break down because we would all too busy hording what we could, plotting against each other, killing each other and having relations with each other.
It's just the nature of man.