It's true, and I feel the same, but to give it some credit: wouldn't that apply to anything you don't use regularly? Programming languages or even regular languages. You have to use it or lose it.
Why not both? GUIs can be built that can emit the command-line equivalent.
In ffmpeg's case, I would be looking for a GUI beyond just "here's all the command line parameters, only in a form"... the hardest thing is taking something I already know what I want to do, and encoding it into the command line, because the way to encode a graph into a command line is not obvious. It is easy in principle but there are so many degrees of freedom in how it is done that I can never remember it all. This is a perfect time for a visual language.
there is winff(not sure of its state today) its totally just a gui frontend even shows you it working in console for an example of something already written. scripting can help with more complicated operations but just for common shorthand there are aliases which can be written out scripts single command lines and pointing to script files.
I use ffmpeg not infrequently, but rarely to do the same thing, so automation doesn't help. Knowing how filter chains work and having access to the filter documentation is great.
Well there are video editing GUIs, even straightforward ones. But they'll have footguns and surprising behaviour everywhere. Lossy or compressed video is a hard problem.
> wouldn't that apply to anything you don't use regularly?
Doesn't apply to well-designed GUI programs. In good GUIs features are discoverable, you don't need prior knowledge or documentation to use software.
Experience speeds things up, e.g. keyboard shortcuts are faster than menus or buttons. That's optional though, one can still use the software without these shortcuts.
There's a continuum, and yes that applies to human languages, both natural and constructed, and to other skills. But where you can choose (so, not natural languages really) you can design things to make this easier, or, I guess, harder.
For example English orthography is a horrible mess. The inventory of squiggles needed to write English isn't too bad, but the correspondence between the words you know and the correct sequence of squiggles to write them is unnecessarily complicated. No benefit accrues to us from this, and in some other written languages it's much easier.
I'd place ffmpeg somewhere in the not-bad but not-great part of the continuum. As with English orthography of course the problem is if you change things with the intent to make them better you actually introduce a cost for existing users which may be impossible to sustain.