One thing that the below assumption doesn't appear to take into account is user demand for quick confirmations. I haven't fully thought out the game theory on this but here goes:
Example: if 75% of hashing power accepts 'medium' fee transactions while 25% is willing to accept low (or any) fee transactions, then a user paying a lower fee wishing to get their transaction included in the next block runs a ~75% chance that their transaction won't be included.
Users desiring the most reliably fast confirmations are better off by paying the minimum fee that a majority of miners will accept. Miners breaking ranks and accepting lower fees only affects users who aren't sufficiently interested in quicker confirmations. As long as a majority of miners 'collude', they will likely be able to keep the average fees higher than miners with the lower costs of operations might be willing to accept.