Grant wrote his autobiography pretty much entirely out of desperation to provide for his family after his death. He had lost everything he had in a Ponzi scheme and was heavily in debt as he was dying from cancer, and the autobiography was his last chance to make money for his family.
That said, at the time of Grant's writing copyright in the US was 28 years (with an optional extension for another 14 if the author lived long enough), which means that OP's proposal of 25 years would likely have been sufficient to motivate Grant.
