If a team's goal is to win the title, they should not be dependent on any particular player. They should definitely keep the replacement. It is impossible to guarantee that a player will play well for an entire season or suffer injury. The Dortmund team is not stable enough this season either. They may qualify for the Champions League, but will be far behind Bayern Munich in the Bundesliga.
Actually, even though they have players who can be said to be a mainstay, but it has no effect on their team's game. The problem is they always let their potential. They don't think for achievements in my opinion, they are just content to sit under Bayern Munich. Because if they wanted to, they certainly could.