The soul isn't a material thing. The only question is what defines the soul not whether or not it exists. It isn't an
argument from ignorance because it has been established that no particular (material) piece of the human body is responsible in and of itself as the quintessential expression of who a person is. It follows that a person has an immaterial component ("the whole is greater than the sum of its parts") and this is by definition the soul ("the spiritual or immaterial part of a human being or animal").
It's existence does not require proof (it's self-evident by definition: the spiritual or immaterial part of a human being or animal). There isn't any "lack of evidence" for the soul. It's the fact that the whole is greater than the sum of its parts that allows us to define the soul.
The soul is a metaphysical concept, like the number one. It doesn't make sense for to go around demanding evidence for the existence of the number one. Scientists aren't going to capture the number one in a jar! There could be one apple or one orange, but neither an apple nor an orange is the number one. We can't call it Argument from Ignorance. It doesn't make sense. The number one is "one" by definition and not because we lack evidence for it.
The existence of wetness is not tied to any particular molecule of water. So it's entirely relevant.
If you disagree, then maybe you should say exactly what the problem is. Are you denying the existence of emergent properties? If so, why?