The Challenger, GTO, 350Z/370Z, Genesis, and Camaro was/were/is supposed to kill Mustang sales. Some may be faster or handle a little better but none "destroy" the Mustang in any performance catagory. The following are just MY opinions as to why the Mustang will not be outsold.
Challenger=fast, but expensive, and big A young driver looks like he's trying to flaunt his dad's car. GTO=fast, ugly ext+int (I think it's cool though), and expensive. European body posing as an American muscle car. 350/370Z=fast, handles great, expensive, and seats 2. ANNOYING exhaust tone...ricer. Genesis=quick, but the name Hyundai will take some time for some to accept as quality although they putting decent vehicle's on market. Trying to copy Nissan/Infiniti. Come up with your own design. Camaro=fast, we'll have to see about pricing. Front end looks like a cartoon character smiling. "Too flashy, I'll get tickets." Mustang=fast, affordable pricing. Durablity. I see tons of Mustangs every day. Some neglected, but all still run hard and still run. Exhaust tone that makes people's eyes follow the car...unique? A car that the owner purchased by choice, not by price (The Genesis may get some sales from Nissan/Infiniti). Anyway, back on topic, the 6 and 8 Camaro's are great performers.