Your conclusion dont have the ability to debunk people's conclusion. Shanks doesnt have to use a sword to use his CoA or CoC infusion to beat people up. He can use his sword, his dick, the air around him, his shoulder blow, whatever. He didnt need his sword and Greenbull's power is deactivated. So its independent power. Nothing you say about Roger's style is swordsmanship change that Roger is also a better haki user than he is a swordsman according to Kaidou. To make haki just an enhancement, Shanks must not be able to use haki without his sword like his warning to Greenbull. He must not be able to use CoC infusion which is an independent power to beat people up. He must not be able to send haki projectiles like in film Red. Which is why your statement of haki unable to be its own power, is your own wishful thinking about it.
Shanks pouring every bit of haki and everything he has in his strongest sword attack and he'd get beaten by Mihawk.
Sure, if you think Shanks has special way of fighting that is irrelevant to swords (and nothing passive, like CoO or CoO killing because that's not direct combat, even Zoro has CoO), then your argument can make sense.
I somehow doubt unarmed Shanks > armed Shanks with sword that he has been using since kid, though.