Like Chinjao, Rayleigh and King implied.
Anyone who uses King's Haki implies they're trying or have the potential to be the best at something.
In order to be the WGS, you would need to have King's Haki which belongs to those with the potential to be the king of something, whether it's Swordsmanship or the world's strongest Pirate.
Furthermore, Haki is a natural ability that all people have, if they're able to use it depends on talent which is no different to being physically strong. Some people have to train to bench press 100kg whereas some people can just bench that easily, and with training end up benching 200 kg or something absurd.
Haki is a part of being human, it's a part of the biology of people. It's something everyone has access to, it's a part of natural strength. There's a reason Zoro trains physically, a reason he trains Haki, a reason he trains technique, everything that contributes to power naturally will be a prerequisite to being the best in anything in One Piece that is physical.