To be fair now the GK Position in this day in age has changed a lot in the last 10-15 years now. These days, Keepers are more adept with the ball and obviously sweeping as well.
The era of old-school Goal Keepering as in just being a Pure shotstopper and nothing else, is gone now. Goalkeepers have to be able to do more now.
I can see why Zubi, at the time, was so invested in bringing in Ter Stegen. He had the tools and potential to be our long term GK, being so good on the ball while also being a modern-day sweeper keeper.
I'm not sure what happened with Mats over the years. He came up Big a number of times in the League, but something just didn't click for him in Europe. He struggled to impose himself, and it just seemed like he couldn't save even weak shots.