One of the problems is that, if Biden does really, really badly to the point that his mental incapacity is obvious, the 25th Amendment might be invoked with support from the Democrats so that a substitute nominee can be put in place. For Trump it is perhaps better for Biden to be the nominee than to have a surprise entrant at the last minute.
or- they purposely over medicate dementia zhao and he collapses on stage, then they are “forced” to replace him...