Of course Namespace in Linux kernel is very mature. But if that's all people use, then there wouldn't be a need for Docker and its extra features- people would

Of course Namespace in Linux kernel is very mature. But if that's all people use, then there wouldn't be a need for Docker and its extra features- people would still be using LXC (no disrespect to LXC). People have to evaluate the entire software as a whole, instead of just looking at the core technology. I personally feel that Docker is still unproven in terms of maturity. Stateless? Hell yes. Stateful app? Well... As you said, a lot of problems would disappear if people use Kubernetes instead of Docker. At the same time, a lot of replication problems would disappear if people use PostgreSQL instead of MySQL. My point is, when a novice mixes immature technology with immature technology, he is going to have more issues than what's necessary.