I agree, more people more volatility like we can't really know what will the price, when will it grow or decrease. More people means bitcoin will last longer. More people means price will unpredictable and the system will grow.
How does more people cause an increase in the volatility of Bitcoin? More people using Bitcoin will cause and bring some kind of stability to the market value of Bitcoin, the price of Bitcoin is determined by the rate of demand and supply. With an increase number of people, both the demand and supply rate will increase causing some kinda equilibrium thereby stabilizing the price.