The same applies for G25 and DFGT, how many people here are using these wheels?No G27 => No PS4 => No GT7. Simple as that.
If you force someone not only to buy a new console but also to change his wheel chances are he keep is wheel and buy a gaming pc instead.
Sony should always remember that lots of people play Gran Turismo because a PS3 + GT6 + using the wheel they already own is still a cheaper combo than a dadicated gaming pc.
But if you start asking 400 € for a PS4 and 500 € for a steering wheel things are going to change.