We want people to be wealthy roughly in proportion to how much they contributed to the economy. The author's heirs can go stand on their own two feet.
Life + 70 is about making it attractive to big firms or other organisations to buy the rights to copyright; it removes the risk of the author dying suddenly. The idea is still absurd, as classics that should be generally available are still under this bizarre lock & key system. Tolkien only died in 1970; his legacy should be part of the public system by now. 1970 was a different time and era. Good stories are dying die because nobody can print mass copies of them.