The strongest part of the argument was the idea of accountability for failure. Since winning the White House and both houses of Congress in 2008, the Democrats have steadily lost ground, not only at the national level but in state and local government as well.
Now we have Republicans in charge of the White House and Congress, in position to nominate lots of federal judges (and probably a couple of more Supreme Court justices), and in control of almost enough states to call a national constitutional convention.
That's an "impressive" record, the kind that reasonably leads to the thought that maybe some new folks should be in charge.
Less impressive is his observation that the last Democrat to become president by election while older than 55 was Woodrow Wilson (Truman and Johnson were older when they won their first elections, but they had already become president by death of their predecessor). At the same time, "before George W. Bush, the last nonincumbent Republican under 55 to win the White House was Herbert Hoover."
Bai says this makes sense because "Democrats win when they embody modernization." But Michael Dukakis had turned 55 just 5 days before the election that he lost to George H.W. Bush in 1988, and he campaigned on his success at having run Massachusetts in an efficient, modern way. Al Gore was 52 when he lost (sort of) to George W. Bush in 2000, and he had been specifically in charge of modernizing government under Bill Clinton.
So youth and modernity hardly guarantee Democratic success.
But the biggest problem with Bai's article is contained in this paragraph:
Anyway, it’s not just that all these iconic Democrats are older; it’s that their vision for the party — with the possible exception of Biden, who’s pro-trade and pro-growth — is relentlessly backward-looking. They’re for government-run health care, expanding Social Security benefits (even for the wealthy) and free college for everyone. They’d pay for all of it with tax increases that magically cover the cost.