This is a great point, and I think lends itself to a better solution than taxes. Why give all the excess to the government, who will do who knows what with it? (actually it's pretty clear where the majority of it goes, and I'm pretty sure most people wouldn't spend it the same way as the government).
If someone attained their wealth with the help of others, why can't we enact regulations saying that the wealth must be divided amongst all of them? Sure you can have some differences for those at the top, but it shouldn't be orders of magnitude difference.
Then each of those individuals would be both better off individually as well as spreading that wealth farther across the economy instead of concentrated to a few individuals.
I'm probably too naive in my thinking here though.