This is not shifting the security responsibility from the firewall entirely — it's just an additional guard rail to ensure only allowed domains can be accessed. It does not mean setting proper firewall rules is no longer a concern, but just continuing WebAssembly's "deny by default" stance on accessing anything outside the sandbox.
We have not explored exactly how large scale apps will look like with Spin and WebAssembly — our initial target, at least in the beginning, is making sure building and running function-as-a-service(-like) applications is a great experience.
That's a great call-out regarding configuration and templates — we do have previous experience with "config template hell" from working in Kubernetes, Helm, CNAB, and other distributed systems platforms, so hopefully we can try to make things better!