In 1776, people didn't know what fascism was. Hell there wasnt even consensus on what capitalism was, Wealth of Nations was published that same year. They had never seen a capitalist system degenerate, as would happen in France under Louis Napoleon in the 1850s.
They knew what feudalism was, which was bad and a form of authoritarian autocracy, but this isn't Fascism. They were afraid that the kings and queens would get restored, as revolutionaries (and capitalism was revolutionary and progressive at that time) they were safeguarding against a counter revolution which would come from monarchists.
There is no way they could conceive of a movement to overthrow capitalism, which they barely understood although being the revolutionary capitalist class, that would come from a greater demand of social reforms, one where the class they were a part of would rule society rather than just administer it as they had for centuries, one where a class that they didn't even know about, the proletarian working class, would supplant them and bring greater prosperity and equality. This movement developed fully in Russia and Europe after the first world war when the last of the weakened feudal aristocracy destroyed their own continent to fight over scraps of colonial internationalism. A revolution in Russia inspired the global working class, especially where they were highly organized and industrialized such as Italy and Germany, and terrified the ruling capitalist classes of those countries.
In the shadow of the emerging workers movement grew the dialectical opposite and evil twin of German and Italian communism: Fascism. Fascists gleefully fight and kill communists, and desire power above all else, exploiting contradictions in liberal democracy (that's "liberal" meaning supports private property, not cool liberals that like freedom and justice) to confuse the masses and gain power. The ruling classes, weakened by decades of militant worker struggles, assented to the will of the fascists and in a last ditch effort to preserve their dwindling control, handed power over to them. The rest is history.
The founders couldn't conceive of the conditions you describe as they either didn't exist or wouldnt be developed enough to study for 50-70 years. Not all forms of authoritarianism are the same. They thought they were doing away with their version of it. Besides, the "founding fathers" gags violently would have fucking loved Trump