I'm assuming that the LH Long-haul flights ex-DUS will eventually migrate to EW, if LH stick to their guns on the 'LH only to/from MUC and FRA' policy.
I hadn't spotted at time of posting that the DUS flight was an evening flight. If it goes double-daily then a morning flight would surely arrive in time to connect to the lunchtime departures to EWR and ORD. There's usually a reasonable chance of getting award seats on those flights, hence the attraction over flying via FRA and MUC.
I wonder if DUS was a more attractive destination than CGN. Surely it wouldn't make sense to serve both?