Will it not be easier to implement new features or even make evolve GCN1 based console when you have the advantage of a low level API access ? And Sony and MS have the possibility to upgrade the APU /GPU in 3 years and release a PS4 v2 and XB One v2 with better GPU ?.. They have allready do it by the past... ( without saying with process advance, they can got some real gain on TDP, performance ).
Add in full hardware/software backward compatibility and almost negligent development costs and 'turbo' versions of PS4 and XB1 in four or five years become very doable indeed.