While I see a lot of derision about Kubernetes these days, if I am starting a Greenfield platform/design/product, why wouldn't I use it?

There are tremendous benefits to K8S. It isn't just hype.

On the flip side, starting out as a monolithic (all in one VM) app will take significant effort to transition to micro services / K8S.

If I think I might end up at microservices/K8S, I think I might as well plan for it (abstractly) initially.