Your first paragraph is a lie; The founding fathers explictly had little trust in the general population to run everything so they created a mixed system based off the Roman Republic.
I'm the one who spams the Farms with huge chunks of Polybius every other week, so you're preaching to the bishop. But we needn't confine ourselves to antiquity; I quoted Madison, Adams, and Franklin in support of the proposition that the US Constitution establishes a government that is founded and dependent upon her people. And I can add Jefferson: "Governments are instituted among Men, deriving their just powers from the consent of the governed." Were they lying, too?
The populace, the electorate, is the ultimate source of authority in the United States. If sufficiently motivated, they can change the Constitution in any way they desire, or enact any law they want. Given a few years to replace enough legislators, the people could amend the Constitution such that every government employee is beheaded at dawn and governance of the country is handed over to a random number generator. (This would be slightly more fair than the current electoral regime.) There's not a thing the government can (legally) do about it, because the Constitution gives that power to the people.
Every US law is passed by the people, indirectly, through their representatives. Every Constitutional office is filled by the people, either directly or indirectly. Every Constitutional officeholder can be removed by the people, directly or indirectly. The legislature is representative of the people, and ultimately answerable to it (or to the people's representatives in state legislatures, in the original form of the Senate); legislators routinely run for re-election and may be removed by their constituents. The executive has authority over that sphere of action over which the people's representatives cannot reasonably act, such as war and foreign affairs; he is elected indirectly by the people, through procedures established by their representatives in state legislatures, and subject to removal if he fails to secure re-election, or if enough legislators decide to remove him. And the judicial power, appointed by the executive with the consent of (half) the legislature, provides expert interpretation of the laws and regulations created by the political branches; any judge can be removed by the legislature. So every office holder encumbers that position at the pleasure of the people, because if enough people get riled up, that office holder will be voted out or impeached. There is not one office, not one person, in the US government whose power does not flow from the people.