Everyone who claims 20 years is more than enough is looking at highly successful works.
Imagine that you are not that popular author whose works are his life savings. Pennies dripping from works you published are your life line that lets you buy food or sustain you in a way.
Without it big publishing companies would just publish stuff without paying royalties, because they could can just do that. With 75 years it will be also some inheritance for authors children - should we also let publishing companies just take that?
There is much more not that successful works that still earn money and need protection from publishing houses than there is "Harry Potters".