they had just had a revolution against a tyrant king. the whole structure of the constitution was to avoid a concentration of powers in a single person or even a single branch of government. they included numbers checks and balances on the powers of each branch, including the president.
billionaires such as the koch brothers, putin, and others corrupted the media and the republican party and then the judiciary and congress and a number of state legislatures to the point where the power structure is no longer effectively the way the constitution envisions it.
today, members of the republican party largely answer to the right-wing power structure, not the electorate, but they control enough of the constitutional institutions to override their original intent and substitute the will of that right-wing power structure.
the framers absolutely foresaw this. they'd be shocked to find it took two and a half centuries to get to this point. i think jefferson thought the constitution would only last 20 years.
they put in a lot of protections against tyranny, but they knew eventually a tyrant would figure out a way to beat the system. i don't think any of them had any illusions about that, they knew way too much about tyranny to think they could prevent it absolutely.