It was never once considered.
So it's finally here, the 2016 Ford Mustang Shelby GT350. And in case you haven't noticed, Ford only unveiled it in coupe form. That's because the coupe will be the only form. Not only is a convertible version not happening, it was also never up for consideration. According to Road and Track, from the get-go Ford wanted the GT350 to be all about one thing: performance. Maximum performance. Cutting off its roof would have diluted that to some extent.
It's logical thinking, and it's the same reason why Chevy won't do a Camaro Z/28 convertible. But what about the now discontinued Shelby GT500 convertible, or the Corvette Z06 convertible? Aren't those two precedents right there? Yes, but it's all about having the right chassis. Like the Z/28, Ford engineers couldn't extract that desired performance from the new Mustang's chassis without a fixed roof. And on a related note, expect for the GT350R, perhaps the Z/28's true enemy, to debut at Detroit. And yes, it too will only come in coupe form.