What happens to 'lost' coins? The BTC supply is not only finite but in fact dwindling over time, as inevitably the amount of currency is going to diminish due to digital loss.
I have personally lost a few BTC due to computer crashes, wallet file lost etc. (In the early days) and considering this the total supply of bitcoins will be diminishing over time at (maybe already?) higher rate then they can be mined.
I haven't taken the time to do ANY form of math on this but I can guarantee that already 10.000s if not 100.000s BTC are lost forever due to people simply not having access to them anymore and the data being destroyed.
Over years of loss this is probably more likely to kill the currency than anything else in my opinion.