First Past the Post voting all but assures that a two-party duopoly persists. What those two parties look like might change over time (just ask the Whigs or Lincoln-era Republicans), but the math of the thing means there’s always going to be two.
This doesn’t jeopardize the two-party duopoly, it just potentially changes the face of the parties to one with greater outsider access.
Which would in some ways be even cooler. 
Whether or not the new face of these parties would be interested in dismantling the two-party system is kind of a separate issue.
