That 5 million is just some number. Could be 20 million or 200 as well.

But eventually there is a point where it doesn't make sense for an individual to own more money. So people start wasting it on huge car collections, five houses etc...

Of course the very rich people are not gonna agree with it because people care most about themselves, that's our nature.
But I am talking about democracy and the few rich people wouldn't have much say in that i guess...