> it's really hard to see how they can be stopped.
I don't think they can "be stopped" - at least not by competitors, as long as they maintain their overall strategy of expanding their marketshare by cutting margins below what their competitors can bear.
In my opinion, this is the way it should be. If a monopoly exists that is more efficient than a competitor can be, then it deserves to exist and its existence benefits consumers. On the other hand, if a monopoly becomes abusive then it will be destroyed very quickly by others who will be more than happy to undercut them.
Amazon (and all monopolies) will fail much more quickly than might be apparent at first glance if they abuse their position. "Bigger" does not always mean "stronger".