There's never going to be three viable parties at once (as you said, not without extensive changes to how voting works, anyway), but that doesn't mean the same two parties are always the major ones. It would be entirely possible for the Republican Party to implode badly enough that the less-extreme ones all defect and we end up with one neoliberal/conservative party and one progressive/socialist party.

in that situation you would temporarily see a uniparty which we know fights with itself to begin with given its big tent philosophy. It would not take very long for it to split. Im thinking you would likely see something like a progressive democratic party split off and the standard democrats would become the de facto conservative party. Could go the other way though.

The Warren Court, from 1953-1969

United States v. O’Brien made burning draft cards in protest illegal

Terry v. Ohio legalized Stop-and-Frisk Police tactics

Swain v. Alabama cemented the All White Jury for another decade.

Walker v. City of Birmingham made the original MLK protest marches illegal.

And unlike the subsequent Burger Court, which began placing substantial constitutional limits on the death penalty, the Warren Court was generally comfortable with capital punishment.

Much of the modern police state finds it's roots in the Warren Court.

Then consider that the first major ruling striking down a law for discriminating against women did not arrive until 1971, after Earl Warren had retired. This was after over sixty years of Sufferegate agitation.

Warren fully neglected LGBTQ rights, despite a number of early cases. Warren even interpreted federal immigration statutes to uphold the exclusion and deportation of individuals based on homosexuality, treating it under provisions barring "psychopaths" or persons with constitutional psychopathic inferiority.

Civil Rights advancements were largely limited to a pool of politically engaged upwardly mobile straight black men. Rights for women and other minority groups (particularly migrants) floundered.

It should further be noted that Warren was a century behind the times, relative to the legislative and even the executive branch. They were still the last branch of government to get behind civil rights, and often in the most minimal fashion.

They could only be called progressive relative to their Hughes Era predecessors, which operated in open defiance of the FDR lead government. By 1941, FDR had thinned the herd of opposition judges enough to consistently uphold his legislative agenda. Stone, Vinson, Warren, and Burger would lead a relatively liberal court for the next 40 years. But it still trailed the national executive and legislatures, which in turn trailed the progressive state governments, which trailed the activist movements nationwide.

The courts are consistently the last to do anything resembling progressive reforms. They are far more often a bullwark against process.