Given that interactions to the cloud are through a relatively small set of known APIs, I'm surprised that there isn't there a service which replicates the APIs but with the endpoints being on "repatriated" hardware.
Designing and implementing a highly available and resilient storage solution on-prem is much more harder task that will take 99% of time & budget to do properly compared to a compatibility layer.
I’m looking for a mechanism which apes existing cloud APIs directly.
But there's also: https://www.eucalyptus.cloud/