CoreOS has a lot of children including Flatcar, RHEL CoreOS, and Fedora CoreOS. Then you have Ubuntu whatever, Google Container-Optimized OS, Amazon Bottlerocket, Talos, and probably a few more. The market is flooded.
The same spirit as CoreOS but it is something entirely different. Written from PID1 up solely for the purposes of running Kubernetes.
I would say that CoreOS is gone because RH destroyed it. Not because people didn't use it.
I think you are misinformed, coreos is still alive within RH products (openshift)