If they drop the Mac Pro line, Mac Pro users will not switch to MacMinis, we will switch to Windows or Linux. We will then in all likelihood stop buying ancillary Apple products.
I would be suprised if Apple couldn't figure that out.
Trust me, they CAN figure it out. Imagine that there was no Mac Pro.
Dell's new slogan would be “Twice as fast, as the fastest Mac".
The Mac Pro is the computer of choice for Hollywood movie makers (Apple's most important and pursued market). Further, while the Mac Pro is used primarily by “power users”, these are the people who influence others on what computer to buy.
As an owner of several Mac pros I have personally influenced over 100 people to switch from Windows to Macintosh. These people generally do not purchase a Mac Pro but one of the other desktop computers.
If the Mac Pro were to go, I would go to Windows.
Power: Although the iMac is an extremely powerful machine in its own right, the Mac Pro's performance still kicks the iMac's butt all the way up and down the block. Benchmark performance in Geekbench shows the 12-core 2.93 GHz Mac Pro coming in with an astounding score of 21,789. That's nearly twice the 11,581 score earned by the most powerful iMac, a quad-core 3.4 GHz model.
Benchmarks only tell part of the story, however. A Mac Pro that's been maxed-out on Apple's online store with as much RAM and hard disk capacity as you can shove into it is a Godzilla of a machine:
• Two 2.93 GHz 6-Core Intel Xeon processors (for a total of 12 cores)
• 8 TB of internal storage
• 64 GB of RAM
• Two ATI Radeon HD 5770 with 1 GB of video RAM -- each.
The best you can do with an iMac via Apple's configuration options?
• 3.4GHz Quad-Core Intel Core i7
• 2 TB HDD + 256 GB SSD
• 16 GB RAM
• AMD Radeon HD 6970M with 2 GB of video RAM
The top-end iMac is an incredibly powerful machine by consumer and even professional standards, but a fully-upgraded Mac Pro is practically ostentatious in the amount of raw processing power it can wield. Professional consumers in areas like 3D rendering, video editing, and other extremely processor-intensive applications surely appreciate the much greater power the Mac Pro can afford them.
Customization:
The Mac Pro stomps the iMac in the customization department. Folding down the Mac Pro's side door gives you easy and almost instant access to its innards, and virtually every component is simple to swap out. Hard drives in particular are extraordinarily easy to swap in the Mac Pro.
Contrast that with the iMac, where the RAM is essentially the only user-serviceable component. Swapping out the hard drive on an iMac is a harrowing procedure that requires removing the entire front display -- not something you're going to want to do more than once, if ever. You could argue that the iMac's Thunderbolt capability vastly expands its customization options (and I will, later on), but it still doesn't quite measure up to the amount of customization available to a Mac Pro.