The C7's not far off and as far as I can see, the C6 is the ugly duckling of the Corvette line.
For the Mustang to be successful, it has to keep selling. This means it has to stay in the public eye and regularly engage the public's imagination. Traditionally, sales tail off towards the end of the production run and there is only so much half a dozen special editions can do especially when dealers gouge the buyer on them. Things like Transformers and Knight Rider will help but ultimately buyers want something different.
With an new Camaro and Challenger both packing hefty V8 capacity and gun power, the "old" Mustang needs more than just a few upgrades to compete against the new kids on the block who seem to be the new beastie boys in comparison.
The Japanese and European car cycles are nearer 5 years anyway so keeping up with the times is all they're doing - although that's a paradox when you're dealing with a "new" retro design.
Besides, it also gives Ford some market research for the new car - it will tell them whether the evolution is going the rightv way, the wrong way, what engine options are going to sell and which won't so when the new car comes along, it will be fresh, new and spot-on. Like the 05 was.
If you follow your logic, the '69 Mustang and Camaros would never have made the cut and would have been the same as the earlier cars. It doesn't affect how desirable the 69's are.