All political parties eventually come to an end.
Most U.S. political buffs have at least a glancing familiarity with the demise of the Whigs in the mid-19th century. But other enduring partisan institutions have also fallen by the wayside--with some frequency, in fact--during our nation's history. (Consider, for instance, the Federalists, the Free Soil Party, the Know Nothings, the list goes on.)
For as long as any living American can attest, however, the Democrats and Republicans have dominated our institutions.
But this equillibrium eventually must give way. As Slate's Reihan Salam points out:
Democrats have a substantial edge with voters under 35 while Republicans are more likely to be on the older side of middle age and septuagenarians. The beauty of the over-65 set is that they are reliable voters. The bad news about them is that they are not long for this Earth.
From almost all accounts, younger voters--particularly Millennials and Gen Zers--favor liberal/democratic causes in a big way. Once the Silent Generation dies off, and the Baby Boomers ride into the sunset, unless voters reallign, the GOP will be in the substantial minority.
However, success is not guaranteed for the Democrats. At least in Europe, many of the traditional social democratic or socialist partiest were supplanted or are at least threatened to be overtaken by new (often populist) movements and parties. Examples include (La République) En Marche ! in France, Podemos in Spain, the Five Star Movement in Italy or Syriza in Greece
For how long will both Democratic and Republican parties exist? Resolution is triggered when:
One of the two parties formally dissolves or is converted into another party of another name, or
Either there are no sitting Republican US congresspeople or no sitting Democratic US congresspeople, or
A US presidential election occurs in which the set of candidates receiving 5% or more of the popular votes does not contain both a Democrat and a Republican.
Note that this question is also in a sense a question on the dissolution of the American political system.