All fine, until all the data ends up stored in the very same Amazon server room, at which point in the event of server fire your "cloud backups" will probably go down along with the original data.
You are misunderstaning how cloud storage works. Amazon/Google data is stored in 3-6 different disks, often in 2 or 3 different datacenters. If that data were lost due to physical failures, you would have much larger worries, like dust from a meteor blocking out the sun.