Well, I think a lot of "brand managers" would disagree with you. Going back to the 60's, Ford/Mustang - Dodge/Challenger - and Chevrolet Camaro - Pontiac Firebird "brand managers" have fought for the biggest share of "the best looking - best technology pony car market." Along the way Dodge and Chevrolet/Pontiac "dropped out" of the "pony car" market and the Mustang took a real drop in market numbers - even though it was the only survivor. The return of the Camaro and Challenger have certainly cut into the Mustang sales numbers. While Dodge and Chevrolet seem to have "gone all in" with new car technology - interior design - performance technology - Ford seems to have relegated the Mustang to "just another Ford" status.
I hope they do well in sales - especially Europe. But I'm not sure that the new Mustang will be very successful in the US "pony car" market. We will understand the performance - technical and economic - in the next few months and years.
While there is still a good market for this type of car and there will probably always be a demand for it this is a totally different sales environment than it was in the 60s, 70s and even into the early 2000s and I think my previous statement still does apply. Times change but I think a lot of people don't want to change with it.
You may want to double check your thoughts on sales numbers.
Mustang sales stayed pretty consistent in the ~120,000-~150,000 range for all but a few years between the early 1980s until the late 2000s. From 2008 on is the first time yearly sales have stayed well below 100,000 for an extended period of time. That also happened to be the time that the economy tanked as well as the reintroduction of the Challenger and Camaro.
As much as I think the 2010-2014 is the best looking Mustang Ford has put out in a very long time I can't deny that it's time for something different.