In a sense, the United States are one-party already, or rather one-government-philosophy. Democratic overtures towards anything that doesn’t overwhelming favor corporates and their investors beyond all else tend to run afoul of right-wing members like Manchin, who every now and then threaten to join the Republicans. This is pretty effective at quailing the Democratic party, it turns out. And they’re already rightward enough as-is that those like Ocasio-Córtez, even with their grudging acceptance of military intervention in places like Syria, still get lashed by them.
The nation will likely still be capitalist either way, so I’m not sure what foundation the complaint has.