I expect commercial interests to take over the role of mining once the number of coins per block diminishes enoughthe value of BTC will define was "diminishes enough" means. Certainly within 20 years, probably sooner.
Mining for transactions fees will become a low margin business, and as such will be handled by corporate interests who can invest enough on the infrastructure side to maximize profits. I could definitely see exchanges (or whatever replaces the exchanges) engaging in this as not only a side business, but a service for their users to ensures fast transactions.
So say I from my Throne of Prognostication.