In a similar vein, and maybe I'm totally ignorant here, but it seems that the end game for bitcoin will be kind of a mess. When the day comes that mining a block only gives the transaction fees, miners won't be actually creating anything. We'll (okay I use "we" very loosely, as I'm only CPU mining for about 3Mhash/S right now) just be shuffling around coins, and unless the price of a coin goes back up drastically and/or the energy cost to mine a block drops drastically, it won't be economically feasible to mine anymore.
So what am I missing?
A few things.
1. Block miners can choose to ignore transactions that won't make them any money.
2. The difficulty of creating a block can be adjusted to be very, very easy or very, very hard. No matter what, a new block will always
be created every 10 minutes or so. If this means making the block very cheap/easy to produce, that can be done.
3. Processing power is always getting cheaper (not that it matters all that much, considering #2). Power-efficient miners (like the FPGA miner project) are gaining popularity as well.
For anybody that thinks there is some huge gaping hole in Bitcoin: Please, try and read as much of the Wiki as you can. Satoshi is a freaking genius, he thought of pretty much everything.