I kind of agree and I think HCI is a stupid marketing fad to sell SDS.
I do agree with you that some of the HCI vendors have good API integration for a proper private cloud strategy, but really that comes down to automation and we could have done that years ago with scripts if we had the appetite. There are going to be customers that go HCI for the private cloud but I believe they are niche and won't drive much HCI demand when you consider the price premium for HCI.
If you want proper automation, self service provisioning, charge back etc, do it propelly and use the public cloud because private cloud offering are imature.
HCI will start to take more of the traditional market but I think a lot of that is down to marketing Hype a not any really technical reasons.