Jamelle Bouie (NY Times) has made a good point about what the "collapse" of American democracy might look like. He cautions against looking to movies, interwar Germany, or the fall of the ancient Roman republic. Instead, he says that you can look at the USA prior to the civil rights movement. At least at the sub-national level, it was the norm that elites entrenched themselves in systems of semi-authoritarian one-party rule. In the South, you had the complete control of the Dixiecrats, backed by violence and Jim Crow repression. In the North, you had big city political machines. Even though presidential elections were typically competitive, the two candidates were basically chosen in private by the local-level elites.
Even today, look at how many states have one-party rule. Illinois has literally had the same Democratic house speaker (Michael Madigan) for almost 40 years!! But at least in Illinois the Democrat's dominance has been backed by popular majorities in elections. In "swing" states like Wisconsin, Pennsylvania, Michigan, and North Carolina, the Republican Party can essentially never be removed from power. Even when they lose the popular vote by a lot, they still hold huge majorities in the legislature. When Democrats win the governorship, the legislature just strips the governor of power.
Based on what happened in 2020, a plausible end point is that Republican-controlled state legislatures start routinely overruling any election they lose, and the Republican-controlled SCOTUS refuses to block the power grab. Republicans typically control around 30/50 state legislatures, which would allow them to perpetually dominate the Presidential electoral college and the US Senate. You'd have a situation where a minority of the US population, living in the poorest states, held complete political domination over the majority of the US population living in the wealthiest states.