Exactly where do "exploitation rights" come from if not copyright? Anyone who is going to enter a contract for these "exploitation rights" is going to want exclusive rights so others can't just share, republish, copy, etc. This is what copyright affords those who want to sell "exploitation rights"

lets say a publisher secures the manuscript after buying it and the copyright is gone at the author's death. As a first mover they will make most of the money during the first publishing round. By the time copycats end up on the market most of the market for the book was captured by the first mover.