> one that doesn't just boil down to "a billion is really really a lot of money".

Why? That's the point. It's too much damn money for one person to realistically earn. We all know how long a day is and how fast a human being can think, move, how much suffering they can tolerate, etc. It is an intuitive truth.

There is no conceivable formulation of physical and mental actions a human could perform, regardless of outcome, that could possibly justify that level of wealth relative to the average levels of compensation for other workers in society.