Having internal expansion does not preclude the use of external expansion. In fact having both 'brings a lot of new possibilities'.
But internal expansions are things from the past now because we're leading to solid slate computer. In a long run, internal expansions won't bring any real benefit, so it's just useless to keep them.
In 5, 10 or 15 years from now, we will the same (or more) power than the current top of the line Mac Pro in a box with the size of the Apple TV. And the more technologies progress, the less average users (including prosumers) will need something more powerful, big towers, etc... Even server farms are now using rack-mounted MacMinis.
It's already the case for some previous heavy duty "Pro users", that can get the job done now with a simple iMac or MBP (sometimes even with an iPad already). 20 years ago we needed the top of the line Desktop computer to run simple application such word, excel, internet browser, etc... something even a smartphone can do now.
There's no doubt, the progress won't stop there and the type of work we currently do on a Mac Pro (and need a high-end Mac Pro for), such 4K video editing, could surely be done on a "non-Pro" device few years from now.
So, it's more interesting to have external connectivity to expansion your system depending your needs... which is something the new Mac Pro has. I even heard about some test done with a computer using 2km fiber optics between CPU and Memory and Graphic card. So, it's just a matter of time before we start to see some external high-end video card.
This is why Apple knows _exactly_ what they're doing and where they're going with the new Mac Pro (a vision that did start with the Apple II, reinforced with the iMac, the Cube, the Mac Mini, the iMac and now the Mac Pro).
I'm even sure that if they could get rid of any connectors and only keep wireless system, they'd do it. That's pretty much what they did with the iPod, iPhone and iPad...
So now, think about that. And if you think you know better than them, just start your own company and we will see who's right 5, 10, 15 or 20 years from now
