Efficiency is only material in terms of something. To provide a devils advocate to your last point, people could be most likely to respond on keyboards because

Efficiency is only material in terms of something. To provide a devils advocate to your last point, people could be most likely to respond on keyboards because a keyboard is an cost - efficient option for the majority of potential responders. However there could be other tech that is more time - efficient. And for someone with an advanced condition such as ALS, it would be extremely inaccurate & inefficient in terms of either cost or time to use a keyboard for any sort of input where a high level of accuracy is required. Whereas a speech-to-text software connected microphone or some other system would be far more efficient in those categories for them. All this to say that no, simply because one person replies to your post with a keyboard adds not one shred of validity to the vast over-generalization that "the keyboard is the most efficient input device we have". I shall also now further assert to, in addition to disproving your central claim by counterexample, to have also disproved your ending statement (also by counterexample), by first, disproving your absurdly general central claim, and second, having done so using a keyboard as the input system for my response.