That’s speculative at best. If full voting rights were given to blacks in the South, segregation couldn’t have survived the election process. I don’t advocate voter disenfranchisement.
I think most libertarians are fine with the federal government having the powers enumerated to it. One of those being protecting the basic rights of citizens from state or local encroachment.
The federal government has never led on any civil rights issue. Never. Some states have always been more enlightened, other states have been more oppressive. With the federal government usually being in between.
I’d turn this discussion around and ask those who prefer a stronger federal government if they would have liked DC’s segregation policy to be required nationwide. Should the federal government’s ban on gay marriage apply nationwide?
The OP’s point would be similar to saying that a Republican (or a Democrat) must favor a strong national government, because after all, there might be states or towns that don’t toe the line in enacting Republican (or Democratic) policies.
A Libertarian, Republican, or a Democrat could all say that the downsides of having such concentrated power outweigh any benefit of having your preferred position be the law nationwide. The argument in many of these threads seem to be that libertarianism won’t work because an official somewhere might propose anti-libertarian policies. The same can be said of any political philosophy.
Is conservatism doomed because New York would never outlaw abortion? Is liberalism doomed because Mississippi won’t legalize gay marriage?