This is Apple's answer to their negligence to the Pro market, and it's hugely disappointing. The iMac was already supposed to be a Pro machine. And then they "New Coke"'d it by making it too thin and susceptible to overheating. Performance suffered and now they're compensating with this machine. At $5000 the iMac Pro falls just short of a ~$2000 Hackintosh. It's still locked into the thin form factor that can't be up…
Nah, this is the stopgap they talked about when they had their "Come to Jesus" meeting with bloggers.
A real big boxy Mac Pro is supposed to come after 2017. Pre-announced, which Apple never does.
See: https://daringfireball.net/2017/04/the_mac_pro_lives
EDIT: They didn't actually say 2018, just "not in 2017"