Nomad replaces parts of K8. It is not a drop-in replacement. If one only want the container orchestration that is fine but then you need Consul for service discovery and so on.
Nomad, Consul and Vault interoperate extremely well and are mostly pleasant to use, but I found myself missing the rest of the ecosystem pretty quickly, especially around ingresses, and I think they made the wrong decision on the networking model compared to Kubernetes.
That said, I haven't played with Consul Connect, the Consul+Envoy service mesh, yet. That might address a lot of the problems. But fundamentally I can't help but think that Nomad and Kubernetes both made a run of it and Kubernetes came out the winner of mindshare and ecosystem.