AWS and competitors are very capital intensive businesses. While the software might be easy-ish to replicate with a medium sized software team, the capital required to set up and then run dozens to hundreds of packed data centers is prohibitive for most competitors. The cloud market will turn into the chip market, where the capital cost of fabrication facilities is an enormous barrier of entry to the market. That is why the only real potential competitors in the space are from companies with lots of cash to burn and who can leverage the same "we need data centers anyways" model that Amazon does.
But it doesn't have to be dozens to hundreds of packed data centers. It just needs to be a lot of smaller operations, and a few mid tier like Linode, providing the same service at various prices and scales that make sense to various sites.
