The danger is that we'll get to the point that >90% of Internet traffic in the US flows via AWS or uses something in AWS. At that point AWS owns the Internet and can start consolidating and shutting out other data centers and providers. It puts concepts like "peering" at risk.
That would be a monopoly. There would be only one way to host, at least for anything serious.
A hard oligopoly scenario with Google, Microsoft, and Amazon would not be much better. We want there to be many smaller players.