Good explanation if you read every single post and email ever made by atoshi but the simple answer: 21 wasn't important, and never was. satoshi guessed at it (not alone).

The idea was to define and maintain a way to get everything running on its own and game theory traveling over time. Make it so supply of new coin is big enough early on for distribution and subsidy, that it contracts enough and at pace with market for miners to wean off subsidy and live on fees. 14 years and counting, it works.

Also, worth noting that 21m came after the idea, perhaps the better question s how the starting number of units was arrived at, as discussed in response above. Equally unimportant and I feel, arbitrary at some point.