Of course, there indeed are numerous political parties in the US, and always have been, but it has generally been a two party system. The two parties, however, have changed over the years, occasionally an upstart party supplanting one of the two established parties. There is no reason to believe that it will always remain Democrat and Republican. I'm sure the Anti-Administration, Federalist, Democratic-Republican, Opposition, Whig, Jacksonian, Anti-Jackson, etc. parties assumed they would stay in power too. These were not obscure parties, but were at one time one of the two major parties.
This polarization was indeed by design, though it was by no means a unanimous decision. Jefferson absolutely hated the electoral college. Hamilton loved it. It does cause the trend toward a two party winner-take-all system, but obviously change of parties in power can and does happen, just at a much attenuated pace. Some, like Torvalds, look at that as a "flaw". Others look at it as a stabilizing factor necessary for long term survival of the republic. Rapid change of political power is not necessarily a good thing. Consider the ease in which the Nazi Party rose to power from the relative political chaos of the Weimar Republic.