I'm in the same boat as you. I purchased a 2008 MP in 2008. Around 2013 I decided that with the way things were going, a quadcore i7 MacMini could replace my 8 core tower, so I waited for the announcement only to be disappointed by the 2 core, soldered monstrosity that they released. I considered buying the 2012 quadcore i7, but wasn't happy with the graphics or the idea of buying a two-year old machine.
So, I decided to keep using my Pro until something better comes along (Which nothing really has)...
The MacPro is upgradable. I went from 4 core 2.8 -> 8 core 2.8 -> 8 core 3.2. I also went from 2 GB ram to 16, added a BlueRay drive, USB 3.0 card, 802.11 AC wireless card, and a PCI mounted SSD drive. Not 2017 tech, but better than it was. I find it boots in 20 seconds or less. I haven't made the jump to Sierra, and am using El Cap. Sierra slowed my MacBook down to the point where it's barely usable anymore, so I am not chomping at the bit to make the upgrade on the Pro.
If there is no MacMini update in 2017, I'll either use my 2008 until it dies (which it shows no signs of) or build a hackintosh. I don't care to buy a computer with built in monitor.
Also, in answer to one of your other questions, I believe you can attach 2 additional displays to the iMac over thunderbolt.