It happens all the time. Contemporary authors have done both well and badly by Jane Austen, whose work has been a jumping-off point for many writers. What it's called is artistic license. Based on your question, then West Side Story should never have been written. In fact, since Shakespeare's Romeo and Juliet was an adaptation of another author's work, then perhaps it should not have been written either. Literature is an ever-evolving field. Words in books are more than etchings on a gravestone.