The main argument against believing the worst is it implies something so monstrous that our minds instinctively rebel against it. As Andy McCarthy put it, lying on the imputed scale would stagger the imagination.
fraud can be so brazen it takes people’s breath away … the individual false statements, sneaky omissions, and deceptive practices… The president’s audacity is bracing, and not just because he lies so casually while looking us in the eye. Obama also insults our intelligence. … To be so bold is to say, in effect, ‘The public is too ignorant and disengaged to catch me, and the press is too deep in my pocket to raise alarms.’”
McCarthy’s observation is interesting because it suggests the existence of a “legend”, a term is used to describe a synthetic identity which replaces an actual biography. It is not about occasional, isolated falsehood but about a comprehensive alternative reality. But like all fictions it depends on superficial plausibility.