I tend to think that we end up with "about 15 years" because it is long enough that nobody can really gainsay you with confidence, but short enough to be scary.

I tend to think that we end up with "about 15 years" because it is long enough that nobody can really gainsay you with confidence, but short enough to be scary. If I tried to start a panic about how [bad thing] would happen in 40 or 50 years, roughly half the population would see that as "long after I'm dead and buried anyway". If I tried to start a panic about how [bad thing] would happen in 3 or 4 years, people would remember my claims when the [bad thing] didn't happen, and my career as a wielder of influence would be over.