You say it "can't be measured", but of course it can be. Say there are 10 million people in your demographic; then your vote makes your demographic 0.0001% bigg

You say it "can't be measured", but of course it can be. Say there are 10 million people in your demographic; then your vote makes your demographic 0.0001% bigger and shifts it correspondingly slightly towards your political position. That's not nothing, that's a small effect. In aggregate, it has a large effect, which corresponds exactly to the sum of the small effects.