Yeah, I agree with you guys. If characters are to have Haki, Oda sets them up pretty nicely. Usopp could see beyond a normal human, he could snipe extremely well, and that is not just about manual dexterity, it's also about visual acuity, the want, and, frankly, the need to see beyond others. Usopp trained in a pretty enclosed island, so he built all of his manual dexterity to the max, here was just the final, and fitting, push.
He was exerting himself, he needed to get that shot. it wasn't just that his friends were going to be endangered or even dead, it's because they would completely disappear from his mind. And that was what scared Usopp the most, after having passed that with Robin, he couldn't go through it again with Luffy. So he needed that last push that couldn't really have happened on his own during the training. He couldn't have appeared already with formed CoO, just the basis of it.
Another character than would most likely get CoO would be Nami. Because she can sense the weather far beyond anyone else. it's not just scientific knowledge, it's a gut feeling, it's intuition. And that's Haki, or at least the basis for it. And it's much more spiritual than Robin's want for knowledge of her surroundings, being a spy, and having the need to understand everything around her, and analyze the threats. But, if we were to give haki to all the characters, then those two would be most likely CoO, and it wouldn't be dissonant from what we know. Then again, CoO is much more person based than actual environment based. Nami is more about natural world, and I don't know if Haki would help out as much. Robin is a bit more person based, but, still, if she were to get CoO it would be because she had to exert herself to find out a piece of information crucial for the lives of the crew, and I don't really see that particular piece of storytelling.