The Fall of Rome was entirely known to them. They tried to temper democracy with wisdom, age, laws, and Constitutional limits on timing, division of powers, age.
But, society has changed, culture has changed. Citizens are not patriots in the same sense. Citizens are stupid well into middle age, and live roughly 25 years longer on average. They are partisan, and the Founders did warn on "factions" as they called parties in their day. Founders could not conceive of the Media Age. This due to psychology, advertising, electronics. These didn't exist in the 1700s.
Lincoln married us to capitalism. Did the Founders know? Women vote now, but they didn't in 1793. Time runs faster now. You can deploy an army in 24 hours. In 1800 it took 6 weeks to 2-3 months. By the time meter alone, Senators, Justices, and the President serve too long. They are afforded too many decades to f*ck things up.
Citizen-legislators of the Artisan class are now career politicians backed by corporate Leviathan's money. Justices are no longer independent arbiters, they are Corporate Courtesans.
Founders set up many filters of various kinds to distill the wisdom of the masses and ensure their cooperation. All the dams have been breeched. They needed to add the Bill of Rights. They should have continued to add the lessons of The Federalist Papers. There are legal, moral, political arguments in there that should be heeded today.
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