Well I'm biased (Khorne player) but it makes sense to get these out of the way when we know their range is pretty complete.
I puts a line in the sand for the foreseeable future, i wouldn't be surprised if we see a Seraphon, Ironjawz, Flesh eaters, and Fireslayers battletome sooner rather than later.
They can always do big releases alongside these updated battletomes.