Well, if the market is mainly for those who used to drive muscle cars like the Cudas, Chargers, and GTOs, why are the new Camaro and Challenger going to have fully independent suspension? It will give both a clear advantage in handling. It's the same market, really. Maybe it's because the baby-boomers who remember the old cars would like to take a GT500 on a twisty road, because the ones they remember couldn't. Also, there's more of a market than those who want to re-live the glory days. While not a major part of the market, there are younger people who like the retro muscle because they never got to experience the they days of the Detroit Iron.
Again, I still say it's an excuse regarding the live axle GT500. Why make the new car just like the old one? The point of redoing it is to make it even better than it was. Who says nobody wants a muscle car that can take turns just as well as current sports car? It's a ridiculous statement.
Ford's official reasoning was that 1) The IRS would cost the consumer an extra $5000 of MSRP, 2) The die-hard drag-racing fans wouldn't buy into a car with an IRS, 3) If they needed to add it later, they could, but they didn't want to do it right away...
...That being said, Ford made the right decision. Granted I've hammered cars like the GT500 for not living up to the performance standards set by its SVT predecessor, but when it comes to the 'lesser' GT and the rest of the pack, the LRA keeps prices down and makes a decent performance model an attractive option at an incredibly attractive price (around $27K for a nicely equipped GT). Certainly it is a 'step backward', but how many people have gone out and bought Mustangs? For God's sake they had to turn customers away in 2005, as they just couldn't build enough to meet the demands of the consumers.
Over at GM and Chrysler, like you noted, they are using IRS setups on the new Camaro and the Challenger. That however is due largely to the fact that the chassis that they had sitting around were designed specifically for the IRS setups, and overall, they didn't have much time or much money to play with the chassis... As the consumers wanted the cars five years ago, not two years from now. The big problem is, however, that GM and Chrysler are having a helluva time keeping costs down on the two new cars, and worse off, trying not to pass these costs over to the consumer. So yes, while we may be getting a Camaro or Challenger with an IRS, base prices are likely to be $3-5K more than that of the Mustang... But, I do see your point on the subject, as it would seem you are getting more for your money on the others overall.
So how do we solve this? We continue to hammer Ford to improve the Mustang chassis with an IRS for the 2009 redesign, and then we move on from there. Chances are that Ford will go for the alternate setup, particularly with the given competition, but we'll have to wait and see for the formal announcement.
...Still, I'm all for the KR. As a die-hard GM guy, it takes a lot to woo me towards Ford, and this Mustang has certainly done so. I'm looking forward to the performance tests, as I would hope that they are better than the current GT500. Not significantly better, but there is always room for improvement...