"Heritage" issues are a modern thing. There were several generations of Mustangs that didn't look like anything old. There's no reason for them to stick with the retro look when the fad is fading quickly.
I have no problem with the Aston Martin looks they are going with lately (for example, Ford Fusion).
Ford has already said the next Mustang is going to be smaller, lighter and will not be as retro so that it can be better marketed as a "world car". It's also been said that all future Ford cars will borrow heavily from the Evos concept. This rendering just puts two and two together.
I nearly pulled the trigger on a 2012 Mustang GT and I'd still consider one, but I tell you, if the 2015 Mustang looks anything like the above, I will be getting one.