Why equal distribution to happen? It is not about just most extreme case, but it seems as unlikely to happen case. Because, I am not seeing any possibilities for all world population to adopt bitcoins at any point before the year of 2100 (in best case and it may take another century of time in worst case scenario).

Moreover, bitcoin is another electronic money and all electronic money are infinitely divisible. It means we are having right now 8 decimals for one bitcoin which can be altered to 10 digits or 25 digits or 100 digits if required.

So, we can cope up with any level of population and its adoptions along with 21 million units of bitcoins itself.