What about dwindling supply?
The current supply will eventually begin to dwindle as people lose their wallet's(removing coins from the system) & governments attempt to drain coins out (either by scamming or creating coins themselves and simply hoarding them)of the system. This is inevitable (government action against BC) as no government worldwide will allow the existence of such an anonymous currency system out of their control or supervision.
That's an excellent point. If this occurs, it might be a good idea to remove or extend the 21M million limit in later versions and freeze the difficulty of generating new coins (by not making it exceedingly difficult to make new ones). Granted, BTC will not be an excellent investment as it is now (because currently we expect its price to multiply in the future), but then again if it turned out to be that popular, the "investment" incentive will not be the main reason people are rushing to use it.