Commodity prices never drop to zero, no matter how abundant they are (assuming a reasonably free market-- government can, of course supply "free" goods, but the results are never pretty). The suppliers of the commodities have to make a profit, or they'll find something else to do.
Not only that, but there will always be non-infinite bandwidth and storage available to users, while anyone can create transaction spam essentially for free. So minimum fees remain necessary for non-priority transactions.