Earlier quoted context omitted.
Laws are usually there for a reason. You could apply your whole argument to public opinion and referendums - public will change mind a lot, why make such drastic moves based on it?
Usually laws are there for a reason. Then sometimes we have to repeal "slavery is OK" or "black people count for 3/5 of white people" or "women can't vote" laws. Because they're obviously wrong.
It was “states that have slavery get to count 3/5 of their enslaved population when determining the representation the free population gets in Congress (and, hence, also it's voting power in Presidential elections.)”
It's actually worse than black people counting as 3/5 of a white person; its more like slaveholders counting as 1 + (slaves held × 3) / 5 people.