Usability is now a hygiene factor in HMI design. You don’t find any specifications for the HMI design, in which the easy-to-use and intuitive interaction is not required. And now Simplicity? Is not that the same?
No, as Giles Colborne explains: Simplicity is more. Simplicity means effortless usage (rather than simple usage), by anyone, without manual and even under chaotic conditions.
Is that even possible? Giles gives us some hints in his highly recommended book “Simple and Usable”. If you have the opportunity to see and hear Giles live, you should definitely attend.