That doesn't change the fact that the market is free and that the transaction fees would have to be competitive. Anyone who charged a transaction fee that's big enough to pay for public services will be undercut by miners who cover their costs and make a modest profit.
The transaction fee and the hashing power of those who charge it would have to be competitive.
Most of the miners aren't solo. They will do what the pool they are in does. If the big pools wants to charge a high fee on transactions, it might be that lower fee transactions take much longer to process. A huge percentage of miners don't seem to want to go solo (or switch pools even) to make more money now, why would they go solo (or switch pools) to make more money in the future?
Right now people are willing to pay these transaction fees to get a transfer confirmed quickly, but there'll be an upper limit. You're right that lower transaction fees will take longer to process. And you're probably right that most miners in a pool aren't going to go solo. But if Deepbit was to increase their transaction fees to over x amount, pools will hopefully break off.
If blocks end up only being created by an organisation that can afford to violently enforce its opinion (ie a government), then bitcoin will have been a failure in my opinion.
