Unless Biden CHOOSES to not run he should be the candidate. Anytime a challenger has run in the primary against a sitting president, that challenge undermined the sitting president, weakening him in the general election to the point of losing.

Reagan challenged Ford in1976.

Kennedy challenged Carter in 1980.

It is of course arguable whether Ford or Carter would have won given other issues, but they were definitely weakened.

Biden running again is not about ego, but keeping the Executive Branch in the hands of Democrats- We may not like all the policies but a GOP win would be even worse - literally a disaster