I've actually contemplated this very idea and think it has merit. What I've found in many applications is that there is a stark distinction between GUI and TUI

I've actually contemplated this very idea and think it has merit. What I've found in many applications is that there is a stark distinction between GUI and TUI/CLI that isn't completely necessary... I speculate that there's more room for grey between the two. To be clear many GUIs can be used via keyboard alone, but often times that ability feels bolted on in that you only get things like shortcuts for a button presses whereas many text oriented interfaces offer more actions which are composites or which might not make sense in a visual presentation of the functionality. Having said that, I think that the inertia of "good enough" is a real danger to the concept. The new user that uses the less efficient GUI aspects of the interface to learn quickly may not have much incentive to go the next step and work the keyboard oriented workflow into their muscle memory. Also, if you realize that issue, there's also the problem that if the keyboard interface were less used, there would be less incentive to fix issues or add features to it as the application changed. Rightfully, it could become less useful because of less attention. I can tell you that in practice I find it fairly uncommon to see staff using the keyboard capabilities insofar as it's already supported by their GUI applications.