Prof. Krugman shares some enlightening information here that the lion's share of income gain is with the 0.1% (the 1% has shown some significant gain, and the top 20% have held steady with a disproportionate income share, but the 0.1% have been on a serious march of increasing income share in the past decade in particular).
I'm sympathetic to the idea that if nothing else, the top 0.1% can be made to pay much more taxes to our collective benefit, using other OECD nations as a reference (for that matter, the top 20% probably can too).
But I think it's a bit silly to call the 300,000 highest earners in the USA an oligarchy. Maybe there's some other better term. Maybe the notion that the USA is becoming an oligarchy rests on some other facts not discussed in his post.
Personally, I think there may be people with 1 in 1,000 talent for wealth accumulation, BUT that doesn't mean that we all wouldn't be better off if a significant fraction of that wealth was redistributed to other projects that would improve our collective welfare.