They likely didn't have a choice, especially as they released a speed bump today. The Mac Pro had become a bit of a joke but the iMac and MBP range was powerful enough (just enough) that the pro community would grumble more than complain.
After the critical reception of the new MacBook Pro range amongst the Apple community, especially pro users, everyone was questioning Apple's commitment to pro users, and the Mac as a whole.
I'm glad to see Apple doing this, but I can't help thinking this was totally reactive and pre-emptive damage control. If Apple had just released the Mac Pro speed bump, there would be even more of an outcry that Apple has given up on anything more than incremental changes.
The pessimist in me thinks that Apple simply had no idea that there was this sort of demand for pro Macs. Phil and Craig mentioned the iMac numerous times, as though to say "hey pros, there is a great Mac you can use", so they can still claim that they do make great, high-performance pro devices.
My guess is "next year" means "Holidays 2018", and my guess is that Apple has only recently started work on this. Apple hasn't been about modular design or expandability for a long time. With the rise in adoption of VR, there has been even more discussion about upgrading graphics cards (remember Oculus comments about the Mac?), and the age of the CPUs is another criticism. I just can't imagine Apple starting work on this some time ago.