[florg]

My favorite related (not a power law distribution) example of this: life expectancy grows with age.

For some people that's really intuitive and others can use it as a more practical stepping stone on the way to understanding what happens in projects.

[Thorrez]

The life example is weaker than the project example.

In the project example, not only does the total project time increase with spent time, but the time left increases with spent time.

[bonoboTP]

That is true for any distribution. The mean (expectation) of the part after X+epsilon has to be larger than after X. There is no possible age distribution where your claim isn't true.