Germany’s defeat in World War I was due to the failures of its military, not the treachery of the politicians back in Berlin. The “stab in the back” was thus a lie, and the military knew it. You can forgive the regular citizens who believed it, however; the Allies never reached German soil, and wartime censorship prevented them from knowing the truth. As far as they could tell, Germany was winning the war. They had no reason to believe otherwise until the armistice was signed.
The “rigged election” lie, like the “stab in the back,” was designed to save face. The difference is that the truth was readily available to the public; it played out in real time. Trump’s supporters refused to believe it simply because they think, based on history and God’s will, they are entitled to rule this country. When the evidence conflicts with the narrative, they go with the narrative every time.
Will the “rigged election” ultimately be as pernicious as the “stab in the back?” Quite possibly. A majority of Republicans have chosen to believe it. It will be a factor in 2022 and 2024. After that, who knows?