.... any perceived Biden acuity issues (if any) will be weighed by the voters.
But, the election is about Trump -- those voting against him vs those sticking with him. Voters who hold strong "negatives" towards Trump vs voters who hold strong "negatives" towards Biden, overwhelmingly favor Biden. [That was not the case in 2016 with HIllary].
Biden is the "adult in the room" contrast for voters who reject corruption, incompetence, and the daily chaos/noise of the Trump Administration.
Trump cannot run on "policy" issues as they change daily, most Americans do not trust him, and the pandemic has exposed Trump. 65% of Americans believe Trump has done a poor job leading us during the pandemic.
If the Trump re-election hinges upon Biden being senile, then Biden only needs to cross that low bar "walk 'n talk" threshold to dispel that challenge.
Even if voters believe Biden is 75% of what he was in 2016, they still have to decide whether to stick with Trump. My view is that the large contingent of voters (like me) will vote for anyone, if it means Trump is gonzo.