GF,
Don't know if I'd refer to the Plane View II cockpit on the G650 as "Old", this is still very much a "state of the art" avionics system, but basically the overall thought to the layout and design of the G650 was made with the intention that it would share a common type rating with the G550/450, unfortunately Gulfstream doesn't have the same clout with the FAA that Boeing does and this idea got nixed, the result being that you have a completely new type, "GVI" for the 650.
From my experience flying the two models I can tell you that the G650 is a very different animal from all previous Gulfstreams, the FBW system is completely "invisible" to the pilot and makes it an absolute pleasure to hand fly, it's ground handling, braking and landing characteristics are by far the best of any previous Gulfstream.
As for the Cabin on the 650, I suspect that Gulfstream wanted to build a cabin that would hands down beat the cabin on their principal competitors, namely Bombardier.
I'm guessing that when it came to the new 500/600 models the thinking was that they wanted to develop a model that would best compete with the latest Falcon models in terms of cabin size/range/efficiency without possibly canibalizing the sales of the 650, hence the slightly smaller cabin cross section along with a smaller, more efficient engines that can still deliver what has now become the trademark cruise speed of Gulfstream, Mach.90.
The new Symmetry flight deck takes the existing Plane View a step further and will also be an easy transition for existing 450/550/650 crews. From what I have been able to gather so far these new 500/600 models will also be a further evolution of the 650 which was itself an evolution of the 550 and so on.