That was what I was thinking originally, as the last Diagrammed day is the 23rd December, and I had assumed that a handful would still be around to be used ad hoc by Control going in to January if needed. But reading the other related thread (455/458 withdrawals?), there is some suggestion that all 455's are off lease by 31st December - this aspect I have no info about, so will not comment, but suffice to say a point (in time) has to come when a line is drawn under this whole matter, where upon those driving the overall plan forward can say, "that's it, the 455s have gone, now everyone can focus completely on getting the remaining 701's in to traffic" (etc).
Sometimes that is what is needed, a kick up the whatsit to focus people's minds, and remove any other distractions - in this case trying to keep a handful of 455s going! Makes sense to me to be rid of 455's by Christmas, rather than let them drag on.