They main reason for switching to the current style keyboard was to save space since they don't travel as much as the previous generation.
There is essentially no real benefit to replacing the keyboard with a touchscreen, it would be more expensive, it would probably increase the footprint, it would provide less physical feedback and and be worse to use and most importantly, it would use significantly more power than current physical keyboards.
A touchscreen keyboard for the entire keyboard is pretty much the antithesis of everything Apple is about.
Today Apple's solid state haptic buttons are so convincing they feel as good as, or even better than, real buttons. In the fullness of time, it is conceivable that they would develop haptics that make a solid state keyboard feel like it's a real keyboard.