I read Capital in the 21st Century. I can't speak to this group in particular, but I think what Picketty argues for is wealth redistribution. Picketty proves that the poor are much worse at investing money than the wealthy -- which is actually the biggest factor behind increasing inequality.

Sane people can argue that taking money from good investors and giving it to bad investors is a bad investment strategy.

But it's quite clear that if we don't do /something/ about inequality, it's only going to get worse and at quickening rate. Wealth redistribution is pretty much the only option.

The only way that seems sustainable to me is: you take the average rate of return and subtract that from each individuals rate of return. If that's a positive value, you're taxed at some percentage of the difference.

I.E. if the average rate of return is 4%, and yours is 90%, you're taxed at 86%*X -- where X is some value or a function of 86%.

If you tax wealth at some arbitrarily progressive rate, that could easily disensentivise people from creating wealth or from working as hard to create it.