This is fundamentally incorrect. See the millions of Photoshop, Visual Studio, Notepad++, GEdit, Eclipse, Microsoft Office, Autodesk, Blender, Visio, Labview, Mathematica, Quickbooks, SAP, Oracle and other users of proprietary application-specific software. Do those users consider themselves to be experts? Do the accountants working at your company consider themselves to be experts? Do you really believe they would be more productive if they had to work in a terminal all day?
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Um, isn’t that particular quote you have there about keyboards vs. mice, rather than command-line vs. GUI? (Next paragraph he talks a bit about one advantage of command-line, but it is a different point than this one).
Expert photoshop users rely quite heavily on the keyboard (keyboard shortcuts specifically), for many operations. The mouse is also used heavily, of course. The keyboard just can’t select portions of an image very effectively, or help you point at a pixel you want different, etc. Whenever I see a Photoshop expert do something to the image, though, they’ve used the keyboard, where beginner users (like me) just use the mouse.
For most of the others, I expect expert X users are similar to expert Photoshop users. Then there are the rest, which are very text-oriented and therefore mostly keyboard-driven by default.
On to the actual GUI-vs-command-line argument.
You say “The way this works in interactive GUI applications is that they provide input forms and status indications to communicate with the user.” His argument was that this is less straightforward than just typing the arguments next to the command. I do not think you have refuted his point.
The simple fact is that both interfaces have their places. I do a lot of work in both, and I think it will pretty much always be that way (for me).
I do not think text-oriented command interfaces will go away while we still read & write, though hopefully they will indeed become friendlier with graphical interfaces.
A GUI does not preclude the use of the keyboard. Keyboards are terrific for entering text and many other things.
" are fundamentally a better input interface for expert operations than the mouse/trackpad/pointer (with a rasterised but basically more analog feel that makes me "look where I'm going")."
This is fundamentally incorrect. See the millions of Photoshop, Visual Studio, Notepad++, GEdit, Eclipse, Microsoft Office, Autodesk, Blender, Visio, Labview, Mathematica, Quickbooks, SAP, Oracle and other users of proprietary application-specific software. Do those users consider themselves to be experts? Do the accountants working at your company consider themselves to be experts? Do you really believe they would be more productive if they had to work in a terminal all day?
"GUIs are great but there are relatively few that have the level of keyboard control that you can get with a terminal-based app."
You suppose that this level of keyboard control is required without providing any basis.
"The second is that text apps let me give commands with arguments much more straightforwardly; there is a language I speak, where GUI-based stuff is more like grunting and pointing. Clicking a mouse on a button or menu item is like calling an argumentless function()."
The way this works in interactive GUI applications is that they provide input forms and status indications to communicate with the user.
"Being able to type your command out, if it's at all complex, is just simply more powerful, and so text-based interfaces aren't going away any time soon."
Textual command systems and scripting languages are frequently integrated into GUIs. Another natural progression though is for frequently used commands to be represented by interactive widgets and dialogs. The fact that interactive graphical controls for many commands are missing often is simply the case because they have not yet been implemented.
I think that there is definitely some work to go in integrating textual language-based communication with visual interactive systems and exactly what form that will take I cannot say. But I think its important not to throw out the visual, two dimensional, interactive and even multi-user collaborative capabilities we have now but rather to integrate them with finer-grained language-oriented features of textual languages.