How is OIDC less clean from a threat/failure model perspective? If your organisation has implemented an OIDC IdP, you can have the API server behind a Bastion

How is OIDC less clean from a threat/failure model perspective? If your organisation has implemented an OIDC IdP, you can have the API server behind a Bastion like you suggest, and accessible through an authenticating reverse proxy (Beyondcorp style) WAF. You can even have token bound access/ID tokens which are backed by hardware TPMs if you're REALLY paranoid (upcoming spec). Plus because it's an open standard which K8s heavily embraces, you get rich group/user claims mapping in K8s allowing for logging and auditing.