Champion Diversity
There are some cases were a champion could reasonably be replaced by a similar one: Yi/Jax, Malphite/Sejuani, Janna/Sona, but there are some others where a champion has a strength that no others have and is more difficult to play around than it is to execute: Ekko/Yasuo/Lee Sin/Ivern/Graves.
If something's overpowered, it shouldn't be okay for everyone to have to ban that champion every game. Both teams should be able to have that, and leave it up to the better players to win the match.
And if this makes the game less diverse, good. There's no reason fairness should be sacrificed for the sake of diversity. It's the unfair nature of a draft that prevents you and the opponent from both having a champion that wasn't banned that leads to things like Kassadin's 90% pick/ban.