You are right that there will be a reversion to the median for individual politicians, but parties definitely change. The republicans have been drifting right since Truman; the democrats can shift left. It will take sustained effort, but every progressive who wins a primary shifts the needle a little.

One caveat - we need to get corporate money out of politics. Like, completely. Corporations aren't people and their donations aren't free speech. There should also be an iron-clad cap on individual donations (no Musk-Rats dumping hundreds of millions on one party). Some countries have publicly-funded elections - a powerful bulwark against corruption.