I'm not sure that white people ever constituted a supermajority of the population, they were just a supermajority of those with rights. I submit there wasn't such a thing as white until after Bacon's Rebellion when it became necessary to separate races. Even then, a lot of people considered white now, weren't then.

By 2045 or whenever white people become a racial minority, don't expect systemic racism to fade away but to become more entrenched. It may or may not be an issue of racism, it may only be about power. But that won't matter because power is held by creating advantages and bases related to race.