[acdha]

That attitude was popular in the 90s but any definition of “sensibly configured” in this century involves a firewall.

The reason is that even experts make mistakes, get busy, or rely on assumptions which turn out to be incorrect. For example, you thought your service which uses strong authentication and encryption was safe to expose – and then Heartbleed or RegreSSHion happened. If you restricted ingress, you slept calmly. If you had it open, you had an emergency rush to patch and look for signs of compromise.