We know this is true because of more than a century of empirical evidence from other "multi-party democracies" (i.e. real democracies). Even in multi-party systems; the vast majority of elections are between two dominant parties. While a few of these systems have splintered into permanent, multi-party coalitions the vast majority have two, major parties. The point, however, is that these two dominant parties change.
In political systems that truly represent multi-party democracies, senior establishment parties rise and fall. This is an inevitable process because of the political reality known as corruption. Power corrupts. Absolute power corrupts absolutely. Inevitably, major parties become terminally afflicted with corruption, to the point where their candidates are shunned by the Voters -- and a new party emerges to take its place.
But not in America. In the United States, Binary Insanity reigns supreme. Unless/until Americans cure themselves of this mental affliction; they will simply elect one government after another bought and paid for by their oligarchs, who laugh all the way to the bank.