One thing that might be interesting to consider is wealth distribution over population growth.

Thought experiment:

Say our money supply is fixed (10 million dollars), and there are 1 million people in our economy. Everyone averages out to holding about 10 dollars. Everyone goes on a massive baby-making spree, and in 10 years, there are still 10 million dollars, but 2 million people in the economy. Average wealth is $5 per person. Either prices dropped (unlikely, demand has doubled in this ideal world), or everyone is averaging to be poorer.

To my mind that thought experiment demonstrates the need for the supply of money to keep pace with population growth.

I might be wrong, I am not an economics student, and there might be angles I've missed.