With the ongoing changes moving so fast it is not always possible to come to the 'best optimum' decision at short notice. It's not just about 455's as units, but also other types in the associated mix (458/450), and crew availability (ie: those crews who still retain 455 traction knowledge), and any stock diagrams that (now) start and finish at Wimbledon depot (from the 455 angle).
There will be other diagrams that run all day that start and finish at Wimbledon depot, but most such stock diagrams will involve crews from other depots at some point during any given day, most of which will now have lost 455 knowledge, and there is just not the time to start re arranging crew diagrams at such short notice, so the planners/controllers will have limited options! That's the reality of such stock transitions.