That idea is nonsense, just look at what GM and Holden did with the Zeta, a clean sheet design.
Their Commodore/G8 Sedan weighs the same as a Falcon but the Camaro is about 400 lb heavier
than the equivalent Mustang. That is why you don't use the one platform for a sedan and a coupe.
Ford has found it more economic to improve costs at the supplier level rather than producing one rather
constrained platform that suits neither car. E8 Falcon is remaining until 2015, I suspect D2C will change
a year or so before that. Development work on a proposed Falcon platform beyond 2015 will be starting
soon but the decision on it does not have to be made until sometime in 2011, plenty of time yet.