*Stuff I don't agree with*
Again, I disagree completely.
Let's do a little spec comparison here (education pricing, as that is what I will be paying).
Current Mac Pro Setup
2.66Ghz W3520 + 12GB + HD4870 + 160GB Intel X25-M
300GB Velociraptor: Windows
2x1TB WD Blacks: Documents & Photo/Video Libraries
1x1TB WD Green: TM
iMac (CPU Performance +40%)
27" 2600K + 2GB GPU + 2TB HD = £1,800
16GB RAM = £70
256GB SSD (So OS X + App + windows can go on it) = £400
TB Enclosure for external drives = There isn't any!?!
So essentially a incomplete iMac setup is the best part of £2270 without any external HD using TB, therefore UTTERLY USELESS.
And if it's useless for a someone who doesn't really push the MP to it's limits then what about people who do!?
Mac Pro 2012 (CPU Performance >+80%)
E5-1650 + Bottom spec GPU + 1TB HD = £2000
16GB RAM = £100
Total: £2100
Plug my current HDs, SSDs, etc straight in and BANG. More performance for less.
So the even the mid (and actually the bottom spec Mac Pro) would outperform the top end iMac (The E5-1620 4 core will outperform the 2600K).
Your comparison on with the Xsan and Xserve is wrong. They were cancelled because they were very very dedicated kit requiring OS X server and a lot of additional server R&D with a even smaller market share. Apple knew other people did this better and therefore left the game.
The Mac Pro is simply there to be the most powerful Mac money can buy. It doesn't require additional software R&D and the R&D on it will be minimal at most. I would bet my iPhone 4 that the 2012 MP will be exactly the same inside except with a updated Mobo and CPU tray due to the change to LGA2011.
You also forget that the MP is what all the developers use to make content for the devices, no development = no platform.
I think you also miss the point that it doesn't have to be a big seller, or at the forefront of Apple's ethos. It's what it is. A powerful, expandable powerhouse which is bought by people that require it's abilities. And ontop of that, if they don't sell Mac Pro's and they are at the core of a workflow then all the other Apple kit (which may be iMacs, MacBooks, iPads) would also be ditched.
It's here to stay. I guarantee it.