Gotta disagree here - the whole point of retro is that it can never "run it's course."
Maybe another way of saying this is "musclecar retro" may have run its course. Whatever the case, manufacturers seem to be moving away from that look. The drawings I've seen for the next Mustang are only a little retro, and the Camaro, Charger, the last Cougar, the last GTO, yadda yadda were never really retro at all.
Ultimately, the retro cars are marketed to a bunch of older folks, mostly men. That demographic is starting to die off. The market for something that both looks and acts like a musclecar may be fast disappearing.