I'm not sure I get this objection. Presumably in a company like Coinbase there is already an infrastructure team that runs the AWS instances, helps build the AMIs, etc. This team could re-tool and hire some k8s experts to help them make the shift. The promise of k8s (at least one of them) is that you can do more with less ops resource, since the system is so programmable. The idea that you'd need a completely new full-time team doesn't grok for me; that new team should replace another team that's no longer needed, or more likely, involve a combination of hiring some experts and retraining your existing engineers.
I do take seriously the other issues raised RE: security (though one GKE cluster per security boundary is a perfectly reasonable approach and gets you further than you might think).
> Unfortunately, neither myself (nor the other data engineer) was a Kubernetes guy
I think this is a different issue than the OP was raising; in any case, in order for a technology to succeed, you need to have subject matter experts embedded in your dev teams, or a separate function that provides the service to the teams that use it.
In the context of the OP, I think your case would be more like saying "hey data team, you need to build your data jobs into AMIs, go figure it out". Regardless of the technology chosen, it's not going to succeed if the teams doing the work don't know how to use the tools.