I agree that human behaviour is unpredictable and irrational, but as far as voting systems go I'm happy to argue that if some change wouldn't affect the result

I agree that human behaviour is unpredictable and irrational, but as far as voting systems go I'm happy to argue that if some change wouldn't affect the result assuming people are voting rationally, then there is no cause to invalidate the vote. Otherwise you're just arguing the vote is invalid because it would have ended differently in a rerun, and you have to stop somewhere.