Except natural selection can't start over. It onlu works if there are always a high rate of survivors and even if that was not an issue consider 4 billion years and a generous generation life of one year (natural selection cycle), 4 billion isn't a whole lot even for small features when you don't have an enormous population and birth rate. Let's say there were 100000 humans at some point and only a 1000 fatal features (being generous) it's not just the replacement rate of defective humans that needs to exceed the elimination rate, a certain percent of replacements must be free of all fatal defects and survive. Also, consider how there should be many failed species that attempted to evolve into a human like species or a primate. You can't always luck out, at some point the entire branch has to fail, requiring subsequent attmepts meanwhile the fatal conditions that required the evolution will not go away.