The new, higher-paying target customers for the political-industrial complex are partisans and special interests. This yields lots of resources, campaign contributions, lobby money - all to support the special interests’ political needs.
Ref:
The votes are essentially already counted (In a duopoly, you don't get to choose what you'd LIKE), which frees the political-industrial complex to focus on other, non-voter customers (The duopoly's true target customers are not the American people).
Because The duopoly can ignore the American people, the parties can structure their rivalry in a way that benefits themselves, as institutions, and are not pressured to structure themselves in a way that would benefit the public interest (The duopoly's true target customers are not the American people).