It takes away from what it really is. The Mustang is a "muscle car" if you put IRS in it, it will cease to be what it is. It'll just be another random sports car on the road. If you want handling, buy a Subaru STI, wonderful cars that handle well. But if they do go to a IRS I'll be looking at other cars then. I love my Mustang and really want another one, but if Ford strays too far away I won't buy another one. This includes the IRS or the "EURO" look that I have seen. It's sooooo UGLY. They need to keep the same body style as the 2014s, they are beautiful.
I think you're misinterpreting the definition of a "sports car". Even with an IRS, the Mustang will not be mistaken for a sports car any more than a Camaro or Challenger would be. Traditional sports cars are about light weight and excellent aerodynamics, and are traditionally two seat vehicles, not 2+2.
While the new Mustang may be lighter and more aerodynamic, it's probably not going to be THAT much lighter than the current generation. Definitely not close to being considered a sports car.
Muscle cars are about raw performance, and I don't see that aspect of the Mustang changing at all. It will still have a big V8 with massive torque in GT form, and a reasonably powerful base V6.
What about a live rear axle appeals to you? I'm just curious. You can't say it's for the track, because it's really only an advantage on a dragstrip and a disadvantage on a road course or racetrack. The new GT500 barely beat out the ZL1 at Laguna in MT's test despite a significant advantage in horsepower and a lighter weight. If it had an IRS, it would have been a complete blowout.
And on regular roads, there's no comparison in terms of driveability and composure.
As long as the styling is good and it still has a "Mustang feel" there is absolutely no reason not to make the mechanical elements like the suspension a little more sophisticated for road use, which is where 99% of people will be using it on a daily basis.