I think we're seeing a paradigm shift here.
For decades it was, more so in the PC sector still: Buy a box that will last many years and just keep swapping the parts out that need to be upgraded. The MacPro "cheese grater" was a godsend in that regard. Never before was it so easy and affordable to keep a top of the line machine competitive with newer offerings. New GPU, new CPU, more RAM, more storage, you could just swap it out like in a PC. Only the mainboard remains a fixpoint, something that PC users have been swapping out for decades, too.
External peripherals on the other hand never lasted very long so far. Mainly because every new generation of PCs came with a new set of interfaces that was faster or totally incompatible with the previous generation. The same was true for the interface cards in the box. They usually died together with the mainboard. ISA to PCI to AGP to PCIe or NuBus to PCI to PCI-X and PCIe in the Macs, your cards never made it over from the last generation of professional workstation.
That's a lot of money down the drain every time you upgraded.
Now it may be going in the opposite direction: you buy an external box, PCIe expansion bays and storage, for example, put your necessary stuff in there and you can keep it for several generations of MacPros, which you will be swapping more often than the old desktops.
Whether that will be a successful shift largely depends on the pricepoint of the new MacPro.
If it's quite a bit cheaper compared to previous offerings, you may end up spending less money over a time of say, 10 years, than you would have before with traditional desktops.
We'll see how Apple decides to do it.
I have a feeling they are going to price it too high, much like the Cube, which incidentally, was the prime reason for it's flop. The extravagant design and the fact that it will be US-made may seem to justify the high price in Apple's view, but it's going to put a lot of people off.
On the other hand, if the entry level MacPro was around the 2000$ mark and would have to be replaced every 2-3 years, while keeping the same external peripherals box(es), it could actually be a pretty economical solution for private and corporate customers alike.