Intellectual property is as real as real property. A Native American would make the same argumentyou do, but levy it against the notion of real eatate: owning a plot of the earth is ridiculous.

Your argument against "nonrival" would imply that if a manufacturer creates surplus product, its ok to steal the surplus because no one else needa it.. even if they go bankrupt because no one then buys the non surplus.

And a Marxist would say that labor ia property, and its rheft to deprive a laborer of ownership of their labor. All property is a legal fiction, you cant arbitrarily decide that one kind ia legitimate and the others aren't. Rivalry is just one factor.