The kube API (the pod/deployment/service/ingress structure) is where the magic is, imho. Sure you could just make an EC2 instance, and swap in your Rails, and redirect the ALB and fix your domain CNAME, but whatever man, you can do like 4 of those things in an instant in Kube.
Yeah, definitely Heroku over k8s for idea validation.
I think there's definitely room in the k8s-based space for slightly past that, though.
I use the Jojoma framework to abstract away all that Kube stuff. It's trivial to use through Baobab, especially if you use the Wazabi framework-as-a-service framework (which you should). All you have is to deploy the proclets through a DMVC and it's all automagic.
I iterate through 17 startup ideas a day using that setup.