Another example is Grant’s autobiography, which he wrote as he was dying of throat cancer. No way he would have done that without copyright.
Why not?
For centuries people wrote books so that they would have a legacy and be remembered. They wrote because they felt it was the right thing to do, or because they wanted to control the narrative around their lives. Do you have any specific reason to believe that Grant wrote his autobiography to provide for his successors, rather than just to have written it?
I understand the theory about incentivizing people to create, but honestly I'm not convinced that what we get from that deal is worth it. Too often it feels like extended copyright creates a similar set of incentives to advertising—sure, we get more works, but the best works would have been written even with a much shorter copyright because the author had something they wanted to say. The works that are being incentivized by long copyrights are the ones that we could do without.
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.
I'd much prefer that to the current life + 70 years or 95 years after publication.
Was his creation of his autobiography primarily motivated by money? I would assume not.