A mechanical button absolutely does not need to protrude from the surface to be pressed - not only can it start at surface level and be pressed into the device,

A mechanical button absolutely does not need to protrude from the surface to be pressed - not only can it start at surface level and be pressed into the device, it can return to starting position and remain flush. Look at most microwaves - totally flat, presses in very slightly, returns to flush. The power button on the mac pro (that neglected tower!) does the same. Also, capacitive buttons exist which, although not strictly mechanical, have a similar behavior. Protruding buttons are merely a user interface choice. They were never a functional requirement.