And BHX-Cologne? Once a day, off peak? Surely the logical thing would be an extra frequency on BHX-Dusseldorf especially given that it's an existing route and the train services from DUS airport to Cologne are so good.
If you'd ever tried driving between the Düsseldorf and Cologne areas you'd realise that the two airports serve different markets. Fly to DUS to reach industrial centres such as Essen, Wuppertal, Duisburg, Moenchen Gladbach, Neuss etc., and CGN if you're looking to reach Cologne (the major commercial centre) and the southern Ruhr conurbations of Leverkusen, Aachen, Bonn and Düren. The market for CGN is going to be smaller businesswise, but for leisure Cologne is always going to win out over the more industrial cities to the north of the Ruhr.
I think they may have pitched it just about right, and if CGN does work, then they can always increase the service to double daily M-F.