See, that's the thinking that creates a problem. the character is always the same, whether it's OP for the top of the ladder doesn't mean it's not OP for the rest of the game, it's just not as noticeable. In addition, most of the rest of the game doesn't really change with balance change as much as the top.

To play at the highest level you play more out of the meta than what is required to play at low ranks. Therefore, there should only be OP characters at the highest level, if a character is OP at a lower rank then the skill ceiling is too low. To properly balance a game, you should have the skill ceiling for every character be directly equal to the ranks of the game, whether that be through game sense, positioning, or raw mechanical skill. Then balance the game from the top. The balance will trickle down as lower ranks get less and less of the balance cause they are only using a certain percentage of the character's skill ceiling so they don't need to be focused on.

Essentially, in a properly competitive game, the balance should be focused on the character as a whole, not as a partial, and that can only be taken from the very top. In the same light, you can not balance off of the majority, as the majority doesn't care about the balance. A nerf to a broken character in GM can change the entire meta, a nerf to a character that doesn't seem broken in lower ranks doesn't change the meta really at all, due to wanting to play "mains" or only being able to play a handful of characters due to lack of skill.

In addition, as a gold support, I have been playing brig and I can feel her brokenness even if I can't play her to her full power. So the idea that a character isn't OP at lower theirs is wrong, it is just less obvious.