Worlds is not the whole of the game. Worlds will always use a small cast of the THE BEST champions. When those are removed, the meta shifts and they use the NEXT BEST champions.
Professionals play the game to win. They get payed to do so. That is their lifestyle. You will NEVER see a champion in worlds if they are not in A) a fairly strong state or B) A player/team has enough experience/knowledge with the champ to make them work.
LCS and Worlds are never a valid argument for balance. Pro's have goldy reflexes and decision making compared to normal players. They will pick a champ who gets results, not because they are fun or interesting or unique.
When you go into ranked, people play the stronger champions because they want to win. If you want a large amount of champion diversity, you need to look to normals.
That said though, there are plenty of champs that are balanced and quite good, but not OP, that don't see play for many different reasons.
Champs like Kalista and Azir need reworks before their playrates/winrates will change positively. Riot is working on Reworking the champs that need it, but it's not an instant process. Even if we skipped the next 4 champion releases, there is no guarantee that we would get 4 more reworks any faster. I almost think it's harder for Riot to rework a champion then it is for them to make new ones.
There will always be champions that are better than others, that is an inevitable fact.