My main point was that there's been plenty of oppression in the US despite it being to some degree a democracy.
It doesn't sound like you're actually disputing my main point at all, but wanting to shift the discussion on to whether the American system of government needs to be replaced and why, which is really off-topic.
Still, in response to your tangential point, I want to make clear that I'm not advocating discarding the entire American system of government, and my dissatisfaction with parts of it as they stand now does not stem from who the founders were.
I do think the system has proven itself to fail at meeting the high ideals that some of the founders professed to have. The system has proven itself to be highly corruptable, the checks and balances built in to the system have failed, and much of the Constitution is widely ignored or reinterpreted to mean whatever the people in power want it to mean.
These failures are not due to the founders owning slaves, but due to them being unable to foresee or adequately prepare the nation for things such as mass media, the internet, modern advertising and propaganda, and a slew of consequences of modern warfare, mutually assured destruction, the military-industrial complex, corporate dominance of the economy, enormous amounts of money being thrown at elections, the shutting out of third party alternatives, the poor civic education, widespread apathy and easy manipulability of the electorate, and on and on.
Despite the founders' short-sightedness and all the fialures and weaknesses in the American system of government, I am not an advocate of eliminating it wholesale. I believe reform is possible, and that it could be made more democratic, more accountable, more fair and just, and we don't have to scrap it all to do it.
However, I very much doubt the political will or consensus is there to make significant positive changes. If anything, I expect it to get much worse before it gets better.. if it ever will.