I think
@Salah WG posted about it before and I remember reading from one of his posts.
I believe in the theory that the original 20 Kings who formed World Govt were the original CoC users.
Then the CD descendents of those Kings grew lazy and spoilt on unearned honor.
Therefore, CoC started being inherited by other people in the verse which scared them so much which is why the World Govt reacted like that when they found out that Luffy was a CoC user.
F.ex., look at Doflamingo.
The moment he came out of the safety of Mariejoa and was thrown into the real world, his will to dominate came out naturally & he unlocked CoC. That ability is in his very blood.
And also, there are still less than 20 confirmed CoC users in the verse, which leaves room for unconfirmed CoC users like Mihawk, Dragon etc,. to be confirmed. But it will never exceed the number of 20 CoC users being alive at any given time imo.