Voters in the seventeen states with the direct initiative, referendum and recall can bury the duopoly via citizen-initiated ballot measures.

Indeed, I've lost track of how many times I've posted links to that podcast in various places.

Unfortunately the legitimacy of this is baked into the constitution, or at least there is nothing to actually prevent the status quo. I don't recall all the conclusions at the end but it will take a lot of effort to break the duopoly and those that could pass legislation to force it are part of the duopoly. So only States and voters can really force it by all introducing ranked-choice-voting for all elections. And of course the majority of Republicans have zero interest in relinquishing what they see as their manifest right to a monopoly on power seized by Gerrymandering, the electoral college, vote suppression etc. And I really don't see the Democrats ever forcing it through the next time they have a majority in all houses - if ever that is.