I know we don't have to worry about this for a very long time, but...
One argument against Bitcoin, and really, digital currencies as a whole, is that, in order to time blocks with reasonable predictability every 10 minutes throughout the life of the system, the resource cost of blocks (e.g. processing power and electricity) increases over time. This system thus assumes that the available processing power will always be increasing and that the cost of electricity necessary to operate that processor will be less than the revenue from block reward plus transaction fees, that mining will be profitable.
If the difficulty never stops increasing, what happens if increased resource costs cause mining to be unprofitable?