Well it's not like luffy ate the devil fruit on purpose. It could happen, there is also a large amount of dumb mermans.
Plus if they got their hands on like an elemental one, they might consider. Imagine being made out of electricty, you could probably bring ships down with ease, not to mention you couldn't be pierced, you could hear peoples plots, etc.
being a merman is a big plus, but it in no way can surpass certain devil fruits
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