And who says this has anything to do with piracy?
If you look at the birth rates in japan, or the total number of young people, the recent part of those curves look almost exactly like the ones for shonen magazines.
It might just be, that their target population is shrinking... because it is:
http://www.nationmaster.com/country/...-_distribution Keep your eye on the 3rd+4th row from bottom. That's basicly their main target audience and it shrinks from about 9.5m to 6m in just 15 years.
How can this not have an impact?
Still this does not mean that you can just extrapolate it and reach zero in 5 years. That is just nonsense.