If Biden does cancel the debate, it will probably be in response to Trump's SCOTUS pick.
Hypothetical draft press release:
"Joe Biden and Kamala Harris are dismayed at Donald Trump's disregard for political norms, the political process, and common decency. Trump's hypocritical and dangerous grab for power is a danger to the very soul of our democracy. As such, Joe Biden has decided to not attend the debate for two reasons: 1. Joe Biden must be free to help Senate Democrats coordinate a plan to stop Trump's efforts in way that honors the memory and dying wish of Justice Ginsburg. 2. Joe Biden refuses to give a platform for the current administration, which has let over 200,000 American needlessly die, to further spread lies and hate. Vice President Biden and Senator Harris seeks to return compassion and normalcy to our Federal Government. Racism and hate have no place in America."