what do you mean maybe, APFS and the new OS is already out being beta tested by the public? Yes the iMac Pro will be introduced for December but that won't be the new iMac we are waiting to see if that's replacing the Mac Pro ( a December release date sure sounds like an 2018 delivery)
I disagree that you buy computer, especially for photography, "when you need them" unless there was an accident or they were stolen. You buy with the idea of longevity because as you pointed out maxed out computer for imaging costs $$$$
The unknown is whether the bugs and headaches with the new system will make a business wish they had bought before the update?