Because any democracy needs a robust and thriving middle class, and we have spent the last 30 or so years transferring trillions of dollars from the middle class to the people at the very top. Just one set of numbers, from the University of California, Berkeley, economist Gabriel Zucman: The 400 richest Americans — the top .00025 percent of the population — now own more of the country’s riches than the 150 million adults in the bottom 60 percent of wealth distribution. The 400’s share has tripled since the 1980s.

This is carnage, plain and simple. No democratic society can let that keep happening and expect to stay a democracy. It will produce a middle and working classes with no sense of security, and when people have no sense that the system is providing them with basic security, they’ll make some odd and desperate choices.

This is obviously not hypothetical. It’s happening. It’s what gave us Mr. Trump (well, that plus the campaign lies). It’s what made Britons vote Leave (well, that plus the campaign lies). It’s what has sparked protests from France to Chile to Lebanon, and it’s what is making the Chinese model — no democracy, but plenty of security — more attractive to a number of developing countries around the world than the American model. Our billionaires ought to ponder this.

I imagine that Mr. Gates is repulsed by Mr. Trump on some level, and at the end of the day probably couldn’t vote for him. But if I could meet Mr. Gates, I’d ask him: Sir, do you not see the link between your vast fortune and the ascendance of Donald Trump? If not, I implore you to connect some dots. Wealth has shifted to the top. It has been taken away from the middle class. That makes people anxious. Anxiety opens the door to demagogues. It’s not complicated.

We need changes in our laws and institutional structures that will alter what economists call pretax distribution. This is a point made by the economist Dean Baker — that income inequality is less a result of tax policy than laws and regulations that have made the rich richer before taxes are even imposed. These changes have to do with monopolies, patents, executive pay and other matters.