Swift,

Here's another aspect:

Lets imagine that there are no transaction fees in 2040.   At that time, there should be strong Bitcoin banks and other financial institutions.  These institution will have every reason to run miner to keep the integrity of their own transactions and to make sure that deposits are safe from attack.  If I'm not mistaken, miners can pick and choose which transactions to include in a block.  I believe Mt Gox runs miners to speed verification of transaction.  Mining helps bitcoin financial institutions even without transaction fees or rewards from new blocks; The incentive already exists.