> grandchildren retaining copyright control over his idea

You are being conservative. Let's try to visualize a hypothetical example...

Age 25 children are born.

Age 40 write work of art.

Age 50 grand children are born.

Age 75 great grand children are born.

Age 80 dies.

100 years after being born. Great great grand children are born.

125 years after being born. Great great great grand children are born.

150 years after being born. Great great great great grand children are born. Copyright runs out.