You're not the only one who think the same most of people in this forum are thinking the same way but let me remind them blockchain isn't a database.Once you put something inside on it you'll never able to re-write it.
You won't be able to delete it, but you can certainly rewrite it. What would prevent you from doing so? You just need to figure out a way to mark old entries "dirty" when updating it - maybe with a reference to the block height where the old value was stored?