The main problem now with DUS is the loss of Air Berlin as a one world partner. Like using DUB to go west, DUS was ideal for those near LCY or who hate LHR. You could check in in under 30 mins and going through DUS allowed connections to all over the world with minimum fuss. That loss has affected loads I would wager and that traffic now seemed to be passing through FRA pre-covid from what I could see.
That said, the city is lovely and hence the loads are still worthy of the route for point to point, esp now FlyBe are no longer in the picture.