John Kerry has been a superb public servant, beginning with his heroic service in Vietnam and extending through a 28-year career in the Senate and a productive four-year stint as secretary of state. He is as qualified as anyone in U.S. history to serve as president of the United States. Yet, he should not run for president in 2020.

Joe Biden is a wise and decent soul, someone committed to making this a better country. In his 36 years in the Senate and eight as vice president for Barack Obama, he too has demonstrated the character and qualifications to be an excellent president. He, too, should not run in 2020.

Sen. Bernie Sanders has provided a powerful voice for the otherwise voiceless. He should not run in 2020. The same is probably true of Sen. Elizabeth Warren. I’m open to hearing the case that she makes for herself, but her botched, naive handling of Native American ancestry questions raises doubt about her ability to handle what comes in a presidential campaign.

Kerry, Sanders and Biden are all 75 or older. Warren is 69. Yet my concerns about their potential candidacies have nothing to do directly with their chronological age. None of the four shows any signs of age-related issues, and they are perfectly capable of significant continued service. But in part because of their age and their long resumes, they will inevitably be identified with an era that seems increasingly irrelevant.

Whatever else it will say about him, history will see Donald Trump as a marker in time that divides what came before him from what will come after him. He is a ridiculous figure, but the fact that he was elected demonstrates the bankruptcy of that previous era.

Hillary Clinton — former first lady, former senator and secretary of state, and also clearly well-qualified to be president — embodied that previous era, which is largely why she lost. She is also a reminder that nobody is owed the presidency or a nomination.

Through incompetence, ignorance and no small degree of malevolence, Trump is hastening the destruction of that previous world. Alliances, networks and understandings that had stood for decades are falling apart, quickly, but the truth is that all that was fated by time to crumble anyway. Wiser leadership would attempt to manage that decline while building replacements, but for the moment that is not the leadership that we have given ourselves.

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Today’s challenges — including climate change, unsustainable economic disparity, technological disruption and a dysfunctional health-care system — are the troubles that must now be addressed. Voters will want and deserve fresh voices and insight into tackling them.

Given the news trend, it’s also dangerous for Democrats to assume that the 72-year-old Trump will even be the GOP candidate in 2020. To cite an example, Biden might defeat Trump handily and would very much enjoy doing so, but a race between Biden and, say, Nikki Haley might produce a very different outcome, for all of the reasons outlined above.

It’s time to move forward, not reach back.

Jay Bookman is a blogger and columnist at The Atlanta Journal-Constitution.