[abandonliberty]

You're assuming no replacement.

I think the calculation runs like this as per[0] 5% => 1/.05 = 20 months average lifespan = 1.67 years => 1/1.67 = 60% yearly churn.

[0]http://en.wikipedia.org/wiki/Churn_rate#Churn_Rate_of_a_Cust...