You have answered your own question. Except, it is the incumbent (Trump) who has been exposed as unfit for office.
It is Trump (the incumbent) who has become the challenger. Biden is already established as the “adult in the room.” Biden’s campaign has been disciplined, ignoring being baited by Trump.
Trump cannot get out of his own way. And yes, it is the presidency. That is the point.
Imagine an incumbent President who cannot defend his own Presidency and hence, was so desperate to attack the challenger, that he tried to bribe Ukraine to discredit Biden resulting in Trump getting impeached.
Imagine that same incumbent President who cannot defend his own Presidency and hence, is now weaponizing the postal service to suppress scores and scores of votes.
America sees it.
Trump is the incumbent. Why does he need Biden to “come out of the basement” and prove himself?