I strongly suspect that they could have gotten Chris Chibnall's first series out in autumn 2017 if they had wanted to, and maybe tided things over with a few 'Specials' in 2016 under Moffat, if they had wanted to.
But the pressing need to reschedule the show was, well, pressing. That Chibnall series could not feasibly be brought any further forward than autumn 2017 so the only option was to push it back to 2018... where they could pursue a spring broadcast. Then presented with the issue of a two year gap between now and then, they manage to get Moffat on board for one more series, which they can deliver by spring 2017. Chibnall is basically missing the first day (or rather, year) of term, and Moffat is the substitute teacher filling in now.
The only problem left is that there's no series in 2016, but that sounds far worse than it is... Series 9 broadcasts right up to the end of 2015, Series 10 broadcasts early on in 2017... the gap isn't too much bigger than it would usually have been and if anything is actually a whole lot smoother and less disruptive than the RTD-Moffat transition which saw a genuine near-two-year gap between Series 4 and Series 5...filled only by a handful of Specials.