Not everything is about defeating Trump. Some of us have a moral compass.
For example, I will not ignore, deny, or downplay an ongoing genocide just because it would suit my presidential preference.
Additionally, now is the best time to attack Biden, because he is a huge underdog after his horrific debate performance. Attacking Biden is pushing the DNC to change candidates before it's too late. Supporting Biden at this juncture is just helping to elect President Trump.