[pdonis]

government action was effective in almost entirely eliminating that form of discrimination

After government action had enabled it in the first place. Jim Crow laws didn't just prevent blacks from riding at the front of the bus: it prevented blacks from starting their own bus companies.

The barriers to entry for operating a bus service would be enormous even without government regulation.

If all the bus services in town discriminate against blacks by making them ride at the back of the bus, but there's no government regulation, a new bus service, even if it's just a single bus, that does not impose that restriction has an immediate customer base: all the blacks that don't want to ride at the back of the bus. (We're assuming that this is a substantial number, which seems historically accurate.) Similar remarks apply to any other product or service where discrimination exists. I find it extremely implausible that, in the absence of government regulation, there would not have been many, many entrepreneurs willing to take advantage of such opportunities, or people to fund them.