K8s probably wouldn't make sense for a lot of startups -- it's designed to solve a lot of different problems, and you're probably not going to be interested in solving most of those.
On the other hand, once you know k8s, it means you don't have to learn the specifics of any cloud platform. For most of my career I've worked in places that can't use cloud providers. I honestly dread having to learn all the different GCP or AWS tools/apis at a future job. But now that I have spent a lot of time learning k8s, it's nice to know that I can build on any cloud platform in the future, or self-hosted servers, and reuse the standard k8s APIs that I use at this job.
If I was building a startup, I'd use whatever the majority of engineers were already familiar with -- k8s, aws, gcp, whatever.