simply because they were born a mere century and a half after the book was written?
How does 120 years equal a century and a half? You have children at 30 when the book was written. Grand-Children 30 years after the book was written, Great-Grand-Children 60 years after, Great-Great-Grand-Children 90 years after, and Great-Great-Great-Grand-Children 120 years after. If you die at 70, copyright would last another 70 years, so your copyright would expire 110 years after the book was written, 10 years before your Great-Great-Great-Grand-Children are born.
"The fundamental principle of science, the definition almost, is this: the sole test of the validity of any idea is experiment." -- Richard P. Feynman