Ask yourself, why Industry is going into stacked, multiple GPUs on single interposer. Ask yourself, how you will be able to fit 40-50 GPU dies in any tower internally? Ask yourself, why Thunderbolt was codeveloped by Intel? Because external expansion IS THE WAY TO GO, to mitigate increased production costs of silicon on smaller nodes. That is very reason why AMD is already loud about scalability of the GPUs. That is why it is important for any OS to see all the GPUs connected to a single computer as a single GPU cluster.
It is the way to go for whole industry. All you people all the time rumble about in this thread and other threads, is dead idea of computing. I have been writing this for 2 years now, since Apple presented Mac Pro. You were not listening, even if every proof that I provided is getting into place(Mantle, HSA, Scalability, external expansion). Small form factor is the way to go for any type of computer. You will have 1-2 GPUs Fury Nano sized, regardless of brand(it is form factor for all HBM GPUs) inside the computer, and if you will want you will get another 40-50 GPUs is a GPU cluster box. You want to know what are 2 key reasons driving this? Efficiency, and production costs.
Mac Pro like computers, even if you like it or not, will be in close future the go-to design. And yes, Apple have made a bad decision. They shown the Mac Pro too early. But right now in, 2016 there is more to think about it, than it was in 2013.