Will the 2 party duopoly ever end?

[Southern Senator North Carolina Yankee]

The biggest factor would be if the existing parties at some point fail to absorb a political movement in substantial form that thus requires it to go outside of the existing tents, but that typically just reshuffles the deck chairs and you end up with a new party system with one of the old ones splitting between the new party and the older former competition (basically what happened in the 1850s).