>The main thing I like is that TUIs are guaranteed to be navigable by keyboard

I don't disagree with you, but in ye old days you had guis that were navigable by keyboard.

And these were discoverable. That one program you use every day, you can spend the time to learn the short cuts. You can still get things done by clicking around though.

I suspect the underlying issue is one of limitations. Terminals are limited in what they can do. Guis and the web, you can do so much more, so people do.

I think another issue is touch screens. Guis and the web have moved to optimise for those. It decreases information density, forces a move away from the old standards to new much weaker ones. Tuis can't really do touch, so they don't have to make those tradeoffs.