I would argue that the keyboard is already the optimal device to both input and manipulate text to the point where touch adds little.

I wasn't clear: I meant that Emacs and Vim are optimized for 10-finger typing on full-sized keyboards. We need something for 2-finger keyboards typical on phones, or more broadly something optimized for phone input (maybe including gestures, for example).

2 fingers is so comparatively terrible that there is nothing on earth you could do to make it better than connecting a keyboard.

VIM actually works pretty well on a phone as long as you find a way to input Ctrl and ESC.

Maybe with evil-mode and some judicious key remapping one could get by.