And unlike gold, which can only be divided up to individual atoms (which would be impossible to handle in practice), bitcoins can be divided with unlimited precision, and paying 2 nanoBTC is exactly as convenient as paying 2 BTC)
So, forget the 21 million limit. There might have been just as well 800 trillion coins, or just 5. By definition, there is an infinite amount of units available.
So at the point where bitcoin is supposed to become deflationary and reward hoarders and early adopters, it can easily stagnate or even inflate?