You've posed a false dichotomoy. I've worked at multiple billion dollar startups that got by just fine without k8s. And I've worked at startups that started on completely different stacks and migrated to k8s. If you're already containerized, you've reserved a good amount of optionality.
The kind of reasoning you pose is similar to "SQL doesn't scale to infinity, so start with an infinitely scalable eventually consistent document store". This line of thinking is dangerous to most companies, and often the death of startups. Assume YAGNI.
There's some truth to "Nobody ever got fired for choosing IBM", but IBM was never a good choice for anyone except consultants. k8s isn't too far from that.