I think you're confusing CLI (an application interface) with a command line. I would argue that CLI applications are inherently more discoverable than GUI applications, if only because `cli --help` or `man cli` give you a pretty complete overview of the functionality; the GUI equivalent is digging through menus and modals--this is getting better with Help->Search, but it's not as nice as `cli --help | grep <feature>`.

I think the real usability issue of CLI is that their potential is handicapped if you don't know how to compose these programs, etc. In other words, if you don't know your way around a Unix command line, you're not going to be able to make use of the tool that assume that knowledge.

I think this is different than what you're saying, but maybe I'm just repackaging your points?