Insert speculation du jour:
Personally, I’ll go with Nancy Birdsall’s useful distinction between destructive and constructive inequality:
inequality is constructive when it creates positive incentives at the micro level. Such inequality reflects differences in individuals’ responses to equal opportunities and is consistent with efficient allocation of resources in an economy. In contrast, destructive inequality reflects privileges for the already rich and blocks potential for productive contributions of the less rich.
That large of an accumulation of wealth at the top is destructive because it can buy that much more privilege for those associated with that wealth.