> You could argue that an onscreen keyboard is significantly less mental load for the pilot than having to scroll letter by letter on a "dumb" interface.

I don't buy that argument. What kind of keyboard? QWERTY or something else? What language? English is the language of aviation, but if you're not a native speaker who uses keyboards all the time then chances are you're going to have a LOT of mental load using a keyboard. Even something like the French or German keyboards, which use mostly the same letters, may be different enough to cause frustration -- and when you have an engine on fire you don't need to be struggling with those details.