Simple is the most misunderstood word in the Agile community.
There are several types of simple:
1) Simple to understand
2) Simple to use
3) Simple in design
4) Simple to expand and/or change
Simple to understand has little to do with anything. Few people understand how radios work, but they are very easy to use. A complex design may make something simple to use. Consider the camera in your phone compared to a camera 40 years ago where you had to consider film speed, shutter timing, aperture setting and focus.
Simple may require more skill to use – consider a unicyle. Also, consider a 27 speed bike with 2 derailers and 2 brakes. Apparently much more complicated than a 1 speed 1 brake bike. But consider how you can set the 27 speed to the speed most fit for a particular person. Then show them how to use the rear derailer, then the 2nd brake, then the front derailer. Easier to use at the start and easy to extend.
Finally, consider a luxury car vs a dune buggy. Which is simpler to drive? Depends upon where you are (i.e., is it fit for use?).
The question to ask is what simple do we want? Simple in and of itself may be irrelevant if it's not forwarding the purpose of what's being used.