Social structures aren’t immutable. But as social theorists like William Sewell Jr. have shown, once structures are locked in place it often takes a highly disruptive event to alter them: a sustained protest movement, a civil war, an economic collapse or a sea change in underlying societal conditions.

For instance, greater acceptance of L.G.B.T.Q. people has arrived only after decades of dramatic protest, litigation and cultural work from activists — combined with opportune circumstances like the decline of traditional religion. And even now the battle for equal rights isn’t close to fully won.

Absent disruptive events, people may try to modify structures — say, by passing legislation or putting in place new policies designed to change how institutions work. But in the usual course of things this will change structures only around the edges.

Which brings us back to the Democratic candidates. The probability is low that any candidate elected as president in 2020 will be in a position to make big structural changes to our society. Changes, yes, and ones that could make a real difference in people’s lives — but not changes that would fundamentally alter America’s social, economic and political structures. (For all the harm he’s done, President Trump has been equally stymied in his attempts at structural change.)

Consider an example from Ms. Warren’s campaign. To shake up the American class system, she has proposed a $500 billion housing plan, to be paid for by estate taxes on those inheriting $7 million or more. The plan would subsidize the construction of affordable housing units, ostensibly lowering rents for lower-income Americans by 10 percent.

The idea is intriguing. But given ideological opposition from Republicans, as well as the political power of the rich — and of landlords and developers who profit from the current situation — it stands almost no chance of becoming law.

If the plan did pass, people at the bottom of the class structure would be left with more money and stable housing arrangements — very good things. Perhaps they could devote some of that money to upward mobility, getting more education for themselves or their children with an eye toward landing better paying jobs. But you would expect people in more privileged class positions to respond to that heightened competition by coming up with new ways to ensure that they and their children stay at the top. Much would have changed in the country, yet the basic contours of the class structure would remain the same.