Riot : Removes Opressor, because not everyone has CC, but then adds Courage of the Colossus

The idea is that oppressor was pretty much the 'correct' choice 100% of the time if you did have CC, and you'd never take one of the other options if you did.

With CotC, it's not necessarily the best choice or the 'correct' choice 100% of the time even for champions with hard CC, as theres other competing keystones that are as attractive or more depending on the champion.