I don't see how this is a problem. There will only be 21 coins in existence and their cost per generation (hence their intrinsic value) will rise up until that last coin is generated. At that point coins will just be subdivided many more times -- nobody will wield around an entire coin on a day to day basis the same way most people don't carry around a roll of $100 bills. Remember coins can be divided down to the 8th decimal place.