Two years for Xcode 9? Don’t think so. It’s likely Apple will require Xcode 9 for AppStore submissions this fall.
For Apple’s ‘Pro’ markets... If the iMac Pro provides a performance improvement that helps Xcode, Final Cut, Adobe users get work done & improves profit margins... Apple will have no trouble selling new iMac Pro systems to these folks.
Of course Xcode9 will be out in the fall.
What takes the time is writing the applications using Xcode9.
From what I’ve seen of the feature set, Xcode9 will need the new high powered Mac’s to run properly.