I think you're right in that Amazon Machine Learning and Azure Machine Learning will never be drop-in replacements for eachother.

However, I would argue that VPSs are already drop-in replacements and I bet (though I have no proof) that VPSs are the majority of the market. The cloud providers may not have a viable way to provide even the perception of vendor lock-in when it comes to VPSs.

Now it could be that prices and profits stay high anyways as the market is an oligopoly. The U.S. has experienced the same thing with mobile operators. This is also far from obvious though because entrance barriers for VPSs don't seem steep.