Ding, ding, ding. This is exactly it. And sure, there are apps that you can reliably kick out and maintain on iMac-class hardware. But the big dogs that you still want sitting at the table (Adobe, Microsoft, Game Devs, etc) look to Mac Pro-class hardware to do their job. And to be blunt, is it cheaper to keep producing Mac Pros, or deal with gaping holes in your 3rd party software lineup? That's a secondary cost one has to keep in mind.
It could even trickle down to iOS. If you stop selling the Mac Pro, and places start switching to Windows workstations, they're more likely to buy a Surface Pro than another iPad. Game developers who don't have the hardware they need to run game engines now might switch to Windows, and do Windows or Android versions first. They've kind of cut their pro lines as much as they can. The Mac Pro is still taking the load for people who were using Xserves, but you cut that last workstation class piece of hardware and things start to get really messy.