From Civil Wars to heart wrenching battles on foreign ground, from economic depressions to the huge industrial and technological boom, and now, having a political atmosphere so polar it is hard for anyone to agree, it is almost shocking that the American government has lasted as long is it has. However, it still stands a individualized but unified country. For a country built on principles purposefully excluding people if you were not white or male, you can expect there to be some major flaws with how our country runs its machine. Our government and land have yet to collapse (though maybe teetering), but the people in our country have become very strongly separated.This gridlock of views can create a temporary peril in the nation as one side can never seem to compromise with the other. This causes the population to be entrenched in what they believe and only that. Compromise is lost, compassion is hard to find, and our country stands on a teeter-totter of collapse. One could say, yes, American democracy is doomed. However, Americans (on some level) are living here right now because they feel stable here. If the quality of their lives are still on the positive side of the scale. American democracy would truly start to fail if the people and the majority of the people rose up and revolted; if they caused complete havoc forcing it to collapse and turn to turmoil.
I also think that the continuing stability of life in the U.S. plays a large role in the persistence of its democratic form of government. Zakaria and Foa and Mounk point out different reasons for the steady erosion of liberal democracy in the U.S. Zakaria points towards the erosion of informal norms that act as buffers against majority rule and towards the rise of entrepreneurship in professional groups. Foa and Mounk mention the inappropriately frequent use of parliamentary procedures and emphasize the divide between the rich and the middle class. The question remains of how to resolve the issue of the polarity of current politics while maintaining a democratic government. As Professor Malekzadeh mentioned, it is very difficult for a third party to gain a following in American politics, so if providing more options is not a solution, then what is?
I think it is interesting that you point out a polar political environment as a sign that democracy might inherently be doomed. In the case of Spain, political polarization certainly played a key role in their adoption of a more autocratic leader rather than their failed more moderate republics back in 1939. However, I also agree life in America is much more stable. Without a much more drastic drop in quality of life, I do not see the people rising up and revolting.