And you can't blame him: why ditch those customers and leave the market open for competitiors...
Don't forget DUS is about the only, and certainly the biggest airport in Nordrhein-Westfalen: an area slightly bigger than Belgium, but with 50% more inhabitants. From this area to FRA or MUC is comparable to Belgian passengers going to AMS or CDG for flights.

So there IS a market for locals flying from DUS to... wherever. It would be crazy to demand more destinations from BRU, but assuming DUS would only be for European destinations for Eurowings. The only thing that would make more sense than keeping Eurowings-flights DUS-NewYork, would be to make those flights LH-flights. But that might make it more expensive (crew-wise), or more complicated (big bases at FRA, MUC, and a smaller one at DUS?).
And besides: if travellers from around DUS go on holidays or a business trip to Barcelona, they use a point-to-point airline, and when going on a citytrip or business trip to New York, they use a point-to-point airline as well... Maybe for such a destination (much more Point-to-point compared to connecting passengers), the differences with the other Eurowings routes are smaller than the differences with LH. Why should a flight to Egypt, Tunisia or Iceland be Eurowings, and a flight to the USA not?