If "private mining" becomes unfeasible/unprofitable, it kind of defeats the original intent of having bitcoins distributed fairly evenly, and not under the control of one or more "power hubs".

Once all coins have been mined the entire mining economy will come from transaction fees. Kind of difficult though to see how private miners will see this as an attractive way to spend their kWh, unless there is a profitable upside. I wonder if the transaction fees will skyrocket in the future to provide incentive for private miners. Having a diverse and distributed mining community is vital for bitcoin I think.

-Michael