The bottom line for Kotkin is that the U.S. economy isn't doing well for younger people. Too many of the young are un- or underemployed, burdened with college debt, and living with their parents.
Consequently, they are looking at "redistribution" - sticking their collective hand in my pocket - to solve their problems. Being almost entirely ignorant of history, they have no idea socialism has demonstrated conclusively it is not a way to share wealth, but only a way to share poverty, to stifle economic growth.
Socialism's Achilles heel is its basis in a mistaken understanding of human nature. It cannot work because it willfully misunderstands human motivation, which in truth is largely selfish.
Idealists don't want to believe most of us, most of the time, want to know "What's the payoff for my family?" If the answer is "Not much, the government will take most of what you earn," expect people to prefer leisure to hard work.
If nearly everyone prefers leisure to hard work, the result is economic stagnation. The economic "pie" to redistribute grows smaller and smaller, instead of larger and larger. As Margaret Thatcher famously said of socialism's core problem, "You run out of other people's money to spend."