It's easy to see the flaws in this reasoning. There are two fundamental errors it.
First, mathematically there is a difference between a tendency towards the marginal price and the price itself. It's the concept of mathematical limits. The limit value of the price is the marginal price (which might be zero), but that is never reached. So the price will never really be zero.
Second, the fact that demand is ont infinite, is ignored. Yes, the product can technically be copied unlimited without marginal costs, but it is technically impossible to actually copy the product infinitely. So the non-marginal part of the price still has to be paid and thus the total price could never be zero.
Third, the original creator of the product has no control over distribution. So anyone is able to distribute the product without having any kind of ownership. It's quite obvious this practice should not be legal.