I think that it all comes down to two observations:
1) We aim at a good precision; recall is irrelevant. In other terms, we must never present an unfree work as being Free, but missing some Free work is of no consequences.
2) 100 years might be a very long time compared to Human lifespan, but it is not compared to typical periods related to copyright.
This being given, it becomes logical to start from a 150 or 170 years figure. -- Rama