This is true, but having a candidate that hasn’t been in power for the last four years at least forces them to invent new attacks rather than just rehearse the same bullshit and substitute “Harris” for “Biden.” Read Trump’s invective. It’s only minorly about his age, it’s mostly about "the state of the country under this administration. "
Regardless of the fact that it’s basically completely false, it is ready-made to be applied to Harris. Buttigieg or Kelly would require new angles of attack on short notice, which could be advantageous for a democratic candidate.