Uh oh! Here we go! Yes the "mustang" out lived the Camaro, but it was also lower cost. One of the things that caused the Camaro to retire was Chevy wasn't as willing as Ford to target the mother figures or people that wanted the car, but not the performance. If that makes sense.
Basically Ford said "ok, lets take the mustang...toss a V6 in it and sell it cheaper." Chevy did it for a bit with the Camaro, but wasn't wanting to keep it up and shame the Camaro. For Shelby going with the Mustang, he did that because Chevy never asked him to make thier cars performers, they didn't need to. Mustangs never out ran Camaro's, Camaro's hands down out performed Mustangs.
As for Shelby...well you take the GT500 or GT350...those arn't mustangs anymore! They are Shelby's! They are hands down one of the worlds finest muscle cars.
But do you know why Shelby did what he did with the Mustang? to quote Mr. Shelby "I only wanted to beat a Corvette." that is why. Now he didn't say Camaro, but point being, the Mustang wasn't a performer.
Hands down, a Factory Mustang won't touch a factory 69 Camaro.
But to be fair, Mustangs aren't bad cars they just arn't my cup of tea. Mustangs have great possibilites, Mr. Shelby proved that.
EDIT: Oh, and as far as the pony car...it has serious troubles surviving, and almost died several times. However Ford pulled off some magic and was able to keep it around. The Chevy Camaro however, never had issues sticking around.