What you fail to realize is that sometimes GPU's are not ideal for all purposes. Some are great for high frame rates in games, some are great for high resolution image or video editing and some are used more like CPU's for use in rendering images of digital models. While the cards that apple put in this thing are most likely good at all of those today, they may or may not be so great in 4 years.
http://www.theverge.com/2013/5/23/4359788/xbox-one-3d-4k-content-support
No, but there will be soon. If MS can make a $400 computer capable of displaying 3d 4k games, its only reasonable to assume that other computer manufactures should keep up.
So you want to have the same computer in 4 years with the option to upgrade just the GPU? In terms of overall computing power, that's not the only thing I hope to be upgrading in 4 years.
My education in engineering supports what I see currently in the PC world. We have really hit the wall in pure CPU speed increase. We are now forced to look at alternative engineering designs to reap significant performance gains.
First, it was all about boosting the clock speed. That wall was hit a number of years ago. And that wall was heat. Can't increase the clock speed without overheating.
Then it became about multiple cores - except the primary problems with multiple cores is that the majority of programming does not take advantage of multiple cores. This isn't marketing, this is software engineering fact.
Now we are pushing the limits of GPU cores to aid the multi core CPUs. And as we do, what's the first thing everyone did with their video cards? Add fans!
Heat!
Apple didn't directly say they were solving a heat issue. But as a person who builds PCs and Hackintoshes, that's the number one issue that every computer case has to deal with. Heat dissipation.
And here for the first time ever is a case design that uses only ONE fan to cool not only the CPU, but 2 high end GPUs.
Have you considered that maybe this is the future core of PCs? Instead of just upgrading 1 video card at a time, we can upgrade a CPU and GPU at the same time?
Isn't that what Intel is trying to offer all the PC world with their onboard GPU which is very low powered against a real video card?
Yes, I will absolutely agree with you that this design throws away the concept of upgrading single video cards and it absolutely blows apart any business work flow that is based on the old workflow that you can upgrade a video card when you see fit.
I'm just suggesting that the PC industry is ripe for a whole new way of designing architecture that will allow us to get past the current walls that heat stops us at with clock speed.