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.
Check your assumptions.
1) We don't know what the future networks will look like.
2) Commodity prices do go to zero for periods of time. Sometimes they rot in silos, and cost money to dispose of them (negative worth).