Hm. I guess I still don't understand.
Fees should be very small, only the cost of including a transaction, which is almost zero because the transactions are only a few hundred bytes of data. Won't the mining difficulty keep going down so that the miners stay just-profitable? Why assume that fees will go up instead of security going down?
Edit: I feel like if I just understood this one last thing I'd really be able to trust bitcoin.
There is a limit on the maximum size of a block. Depending on the size of the transactions, most blocks will hold about 4,200 transactions right now. The miners (or pools) are allowed to choose which transactions they include in the block they mine. If there are more than 4,200 transactions every 10 minutes, the miners will eventually choose the most profitable transactions to include (the transactions that have the highest fee per byte). The rest of the transactions will have to wait for some other miner to pick them up later. Anyone who wants their transactions confirmed quickly will essentially be bidding for space in the next block. This will drive up the transaction fees until an equilibrium is reached.
The increase in fees will encourage more miners, which increases security.
Keep in mind that the block subsidy will continue for 136 more years, so you are getting concerned about something that probably won't happen in your lifetime. Trying to predict what technology will be like and how people will deal with the complete loss of the mining subsidy right now is kind of absurd. Can you imagine asking someone in 1877 about their predictions for technology in 2013?
