I could just as easily come up with an alternative narrative where individuals don’t value political parity that highly due to the initial equal distribution of voting rights, they can expect other voters to have similar interests. By the time the attacker has bought up a large share of voting rights, it’s too late to do anything.
Also, you don’t know that social norms would create transaction costs higher than the benefit to the attacker. You’re speculating. That isn’t evidence.