To get "per year" instead of "per however long a programmer remains in the industry". Suppose there are X programmers in the US who could plausibly start a star

To get "per year" instead of "per however long a programmer remains in the industry". Suppose there are X programmers in the US who could plausibly start a startup. All X people can't start companies when they're 22, and again when they're 23, and again when they're 24, etc. (the vast majority of founders only ever start one company). So, if you assume that 1/15th of the programming population starts when they're 22, 1/15th starts when they're 23, etc., the total fraction of people who start in any given year is 1/15 * 1/15 + 1/15 * 1/15 + 1/15 * 1/15 + ... = 15 * (1/15 * 1/15) = 1/15th.