Why are you comparing a 5 year old (old model) highly modified car to a brand new one? (don't get me wrong, that 08 is a beast, and an awesome car)
The better question would be why would you buy the RS40 over say a very low KM still under warranty current model STI when they are the same price.
2011 Subaru Impreza G3 WRX STi MY11 All Wheel Drive
No doubt they will still sell all 300 of them though. The car just sell's itself. People looking to buy a new WRX would be easily convinced to drop an extra few K for a "special edition"