It’s the classic playground retort, “I know you are, but what am I?” Everyone knows that Biden has dementia. It’s obvious to anyone who’s ever known an elderly person with cognitive impairment. Someone (not Joe) came up with the bright idea of Joe trying to turn the tables and accuse Trump of being cognitively impaired. It’s laughable because, say what you want about Trump, his brain is firing on all cylinders. The Biden campaign can try tactics like this, but the truth will come out for all to see at the debates... if there are debates, that is.
Your answer has all the elements why there won’t be a presidential debate. If Biden does not pass away, there will be a made up reason for him not to show up (got to keep the wuhan flu restrictions going) and the major media propaganda arm of the demonkraps will dutifully blame Trump. But why not, here is a guy who until 2016 was never involved in running government, yet he is responsible for all the systemic racism. The sad thing is how may American fools believe lies.