I see where you are coming from, but I simply can't agree with your reasoning. As I see it, the only thing that can follow from the fact that a majority of people think something is art, is the fact that a majority of people think it is art. What tells us if it really IS art is if it adheres to our definition (idea) of art - that's just the way our language is constructed. Like I said, there simply isn't a correlation between the opinion of the masses and facts - all the more so in this case since a subjective matter like this only makes it pointless to talk about facts.
To put it another way: A solipsist would certainly not care about what other people think is art - he doesn't even believe they exist. And wether they exist or not, their opinions on art would not render his obsolete - after all, there is no such thing as a "faulty" opinion.
Perhaps you could argue that something is art "from humanity's perspective" if a majority of all humans agreed upon it, but I think circumstances would have affected that decision so much that it's pointless to talk about it as a "natural" perspective of any value.
