How many times has Cinderella, or little Red Riding Hood, or the Little Mermaid been rewritten? Fairy tales and myths are a wonderful toy shop that writers can run around in, take things off the shelves, play with them for a bit and then put them back for someone else to have a turn with. All of these stories are in the public domain which means that no one owns the copyright to them. You can take whichever character you want, completely change them, and put them in a new and exciting environment. It's a great way for writers to experiment and have a little fun.
I don't think it's unoriginal to take these stories and to try and make them your own. Yes, people have heard of them before. So what? Should stories come with a shelf life? Should we not be able to write about them after a certain number of years have passed? Yes, some fairy tales have been done to death, like King Arthur. But that does not mean that no one should ever write about him again. It just means that in a market already flooded with King Arthur stories if you want to have a go at that myth you will have to do a lot of research to figure out how you can make your story fresh and stand out.
At the end of the day, I write for my own enjoyment, which I think is a good thing. So what is the point of rewriting old myths? For fun. It interests me. I think the stories are fascinating, the world rich and colourful, and I love putting my own spin on it.