The fact that Mac referred to harpsichords as part of his reasoning leads me to believe that he is thinking in terms of cycles that span longer periods than, say, a short term commercial trend like the Hula hoop or slinky.

Its pretty hard to refute that all things are cyclical. The only point left unanswered is how long the current cycle has left. Could be generations, could be mere years.
I don't think guitars will vanish in our lifetime, but I bet they won't be the most commonly played instrument in popular music 100 years from now.