In a rational world with perfect information, yes, limiting the supply would help the artist. The world is not strictly rational. As someone with family in the book business, I can promise you that there are tons of irrational buyers.

Book buyers will buy because they see other people reading that authors books (more copies in the world means more sales in this case), they will buy special editions of their favorite books they already own, they will buy copies of books that they intend to read once and give to a friend even when the library is a block away and has copies, they will pay extra to have a hardcover new release even though there are effectively infinite other books that could entertain them until the paperback is released. Rational economic choices are actually not a given at the consumer level.

I fully agree about copyright. As a wild example, unless jk Rowling dies in the next three years, Harry Potter will have US copyright that started in the 20th century, and extends into the 22nd century.