Because that contradicts your previous paragraph.
You claimed this: "People with Haki can be a Swordsman, but only if the Haki contribute less than their swordsmanship, in regards to their power."
Going by your example, you'd be saying this: "People who Sing can be an Actor, but only if their Singing contribute less than their Acting, in regards to their talent".
Which is not true, because Ariana is both a singer and an actress. Meaning even if someone Shanks was better at haki skills than at sword skills, he'd still be a swordsman.
False equivalence, and you didn't tie it to the second sentence for context.
X can have singing skills and acting skills. But because X's singing skills is better than the acting skill, then X would be Singer first, Actor second.
In regard to the discussion of this topic as a whole,
- A can have Haki skills and Swordsmanship skills, but because A's Haki skills is better/stronger/more prevalent/whatever, then A is "Hakiman" first and "Swordsman" second.
- B can have Haki skills and Swordsmanship skills, but because A's sword skills is better/stronger/more prevalent/whatever, then A is "Swordsman" first and "Hakiman" second.
So,
- Ariana is a singer, who can act.
- Jackie Chan is an actor, who can sing.
And thus :
- Zoro is a Swordsman, who can use Haki <--- correct
- Zoro is a Hakiman, who can use sword <--- incorrect