Berlin
What many don't realize is that Berlin is essentially a leisure route especially from the regions. In fairness it almost seems counter-intuitive.
Just because it is Germany's capital and its biggest city doesn't mean the yield is there.
The gov2gov market makes the Berlin to [insert other capital city] market worthwhile but outside that its a low yield tourist route.
Those kinds of loads on a low yield destination are going to mean only one thing and that is what has happened here.
DUS on the other hand is a totally different proposition as it is the centre of the Rhine Ruhr metropolis and I believe to be Germany's richest city.
If BHX were to lose all its international routes, I wouldn't mind betting that DUS would be one of the last to go. So nothing to worry about in terms of low load factors.
As so many pprune posters continuously say, it's the yield - ie how much money can be made - that really counts.