Many companies are successfully managing their own on-premise infra to save costs. It's not anything new or rocket science, this is how stuff was before the cloud in the last 15-20 years.
What risks do you see that aren't there on the cloud and meaningful enough to not enjoy the cost savings? Scaling? People have solved for that. Orchestration? Kubernetes works with baremetal and on-premise too. Redundancy is cheaper to ensure. I can name many more.
DHH has done god's work in spreading the gospel of moving back to on-premise. It would be interesting to see where this trend goes.