All these arguments about the Senate being undemocratic miss a central feature of American politics: that the states are popular and legitimate units of government in their own right. If we apply one-man-one-vote, then of course the Senate is wildly undemocratic. But if we look at the instutition in the context of the United States as a federal entitiy and that is not really the case.
Since states are "sovereign" in the sense that they are not merely departments made by Washington and command a democratic legitimacy of their own (by being older than the current governement and having their own elections), it is only proper that they have a separate and powerful voice in Congress. If America were a unitary state and there were just a unicameral parliament in DC then the MP from Delmarva would have no real influence, while the MPs from the metropolis centered on Manhattan and from other cities would have near total control. Maybe that's not too bad an idea. It certainly works well enough for some countries. But America, being a federal entity even before independence, cannot and will not ever operate that way.
Basically, the Senate represents the people through their states, while the House represents them en masse. But even in the House the inherent legitimacy of states as popular units of government is apparent. Congressional districts cannot cross state lines, even when it would seen to make more sense to do so (like around New York and New Jersey, or Chicago and Gary, Indiana).
Obviously, this could all be reformed away with a single constitutional amendment. But keep in mind you'd be going against hundreds of years of political culture. Not exactly a recipie for success in a world where we can't ever get Chained CPI to happen.