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.

Yes, it seems almost all politicians begin to "rot" once elected, but that's why they need regular pruning. New blood, new ideas, and energy for every cycle. It is exhausting but necessary.