GT5 NASCAR and EA Sports / Activision-Eutechnyx NASCAR (the licensed versions) are going to be two different things altogether. Whereas GT5 focuses more on the racing aspect, the licensed versions focus on the NASCAR experience as a whole, from racing to "Rick Hendrick - the Experience." The physics for the games have always been a little suspect when compared to GT5 as well. You're going to see people fitting into two different categories:
The people in it for the racing and competition - they'll probably stay here.
The people in it for the NASCAR experience - they'll probably flee to the licensed version first chance they get.
The licensed version brings with it all the little technicalities that GT5 wouldn't be implementing anytime soon, such as cautions, terminal car damage, and the championship format, not to mention the NASCAR tracks that GT5 hasn't/won't implement as well. So yeah, there'll probably be a significant dip in NASCAR rooms when the game first comes out, but it should level back off shortly afterwards as people begin to decide which game they like better.