It is a catch 22 for Biden he doesn't debate Trump - he looks weak. They hit him on being old and weak. He debates Trump - Trump look like a more legitimate candidate and Trump get to hijack the debate to spew bs.
Not debating is probably the better of the two but both is a loss. The best Biden can do is try to say that Trump won't debate on good faith but that despite being true will only get so much traction.