The original I Am Legend's ending retcon allows I Am Legend 2 to fix another major original movie mistake. Based on Richard Matheson's novel of the same name, the original I Am Legend movie stays true to many elements of its source's storyline. Like the novel, it focuses on Dr. Robert Neville's post-apocalyptic adventure and his struggles with finding a cure for a mysterious disease that has turned the majority of the human population into monstrous creatures dubbed Darkseekers. However, after staying true to most aspects of its source, I Am Legend significantly changes in its closing moments.
https://screenrant.com/i-am-legend-2-ending-retcon-zombie-vampire-mistake-fix/