I am not disputing that AMD could come up with something huge (I think and hope for everyones sake that they do) I think if they are going to 20nm a complete redesign is also in order. Anything to do with the HD 7970 in GPU terms is ancient history and things have moved on. I am sure we are going to see an entirely new and probably more efficient layout.
Their entire future HSA, GPU, APU business model is built around the GCN design, everything is geared and designed with that, and GCN is certainly not broken, its very good and AMD are very happy with it.
So why change it? if it isn't broke don't fix it. especially at this stage.