My 2 cents on the Mustang is that the base price is now around $23,000 when in 2005 it was $19,000. 2010 was the time of the car sales crash and the Mustang'e restyle and price increase. (The new interior was passed on to the customer.) There were many great features added to the Mustang like the new engines and all, but the customers still expected the same price.
Then it also got competition.
Combine that with it having the same basic shape from 2005 on, (A shape that I personally still find beautiful.) and I think you have the formula for the sales decrease. I hope the new generation changes that. And there should be a base model with a... dare I say it... a turbo 4.