> In addition they cause other difficulties, like testability, i18n, l10n and accessability.

Most TUIs don’t have these either. So I don’t see this as a difference between TUI/GUI. If you want to make a GUI and want to ignore these things, you are free to do so.

Where I think TUIs had a niche GUIs don't quite reproduce is in the very particular way DOS TUIs processed input.

An old school DOS TUI reads keyboard input one character at a time from a buffer, doesn't clear the buffer in between screens, and is ideally laid out such that a good part of the input is guaranteed to be fixed for a given operation. They also were built without mouse usage.

So an operator can hammer out a sequence like "ArrowDown, ArrowDown, ENTER, Y, ENTER, John Smith, ENTER" and even if the system is too slow to keep up with the input, it still works perfectly.

Modern GUIs almost never make this work near as well. You need to reach for the mouse, input during delays gets lost, the UI may not be perfectly predictable, sometimes the UI may even shift around while things are loading. Then also neither does Linux, I find that the user experience on DOS was far better than with ncurses apps that have all kinds of weirdness.