Remember ford had no competition from the Camaro or the challenger for a lot of years,sales had to suffer lately.
Even when they HAD competition from the Camaro, they sold well - better than they do now.
This should be the final nail in the retro styling craze. It's evident the sales drop started after the novelty wore off the retro styled car. Everyone who wanted a Mustang that looked like an updated 60s car bought one when they first came out. Car guys love them, but the general car buying public obviously does not. Classic car people are a very small part of the market.
Also, the lack of a sophisticated suspension hurts the Mustang. For the longest time they were the only game in town when it came to powerful rear wheel drive front engine cars, but now car companies are coming out with their own versions of front engine rear drive - the FRS, Genesis, 370Z - all of which out handle the Mustang hands down. Ford has no choice but to change the design into something people want. Retro is cool but it has a very limited market appeal and that market has been saturated.
Also, the Mustang does not sell well overseas. The economic reality is they have got to have a car that DOES sell overseas, and a big heavy car that lumbers through the turns may be a, American car guy's dream but no one else in the world is really interested in that.
I think the new Mustang looks cool and if they finally give it a rear suspension that didn't come out of a tractor, I would seriously look at one.