The presence of minor parties on the ballot doesn’t “place immense pressure on the duopoly”—it just tips the balance toward one or the other component of the duopoly. Which is why either party will actively encourage it when it suits them.
Edit: There’s a historically-proven method of forming new parties in the U.S., which is why we don’t still have the Whigs or the Federalists. In the past, distinct factions would form within one of the dominant parties, until the parent party imploded and two or more new parties emerged. That process of internal fission was suppressed after the Civil War, and that’s how the “duopoly” now maintains its power.
Of course, a different voting system would serve the same purpose (arguably better), and the suppression of alternate voting methods is also duopolistic. But the existence of minor parties under the current system just reenforces the duopoly by channeling dissent away from internal factions.