"Do you think that the art form of dance, which is probably the oldest art form humanity has created, is inferior to painting or other artforms?"
I doubt anyone can accurately define what the origin of "dancing" is, but it's probably something to do with primitive, tribal type stuff involving war dances, celebration of war victory, etc. I don't really see it as art. To take it from that tribal level and drag it into some type of ballroom dancing thing seems like a distortion of it's roots and what it's function is.
This is why people are more prone to dance while drunk, because nobody even knows the point of what they're doing. Once you drag it away from those roots, it turns into more of some weird vanity thing of trying to get people to look at you and admire you. Women are more prone to this type of stuff, which is why most men in modern civilization either can't or don't want to dance really.
You can still see black people from "the hood" utilize it in it's native form doing things like overly excessive NFL touchdown dances that last 10 minutes in order to try and convince their opponents they are superior in the imaginary war, though. Or those Maori soccer people.
"Do you think that human vocals are an inferior instrument to other instruments?"
From an objective standpoint, if you're trying to encompass all music - probably, because anything can be classified as an instrument like reeds in a marsh or something whacking around, to the point where you could say white noise is music.
There's more variety since it's basically unbounded complexity in both analog range while also not being limited to human language. Voice is analog, but unless you're rambling incoherent stuff, it's going to be more limited and repetitive. It's like comparing an open entropy and closed entropy system. But that's only from an objective standpoint. Unless you take the stance the purpose of music is to be like some type of bard and express a specific idea.