If Borussia Dortmund had not sold their players in the last 2 seasons, they would probably be the biggest championship candidate this year. If Bayern Munich becomes champion every year, Borussia Dortmund, which sells its best players every year, also has an impact on this. However, they somehow manage to fill the shoes of those players. Dortmund is a different team.
We know bundesliga to be a tough league and it belongs Bayern Munich because it's their birthright because they claimed ownership of the league with full dominance in games and winning bundesliga titles consecutively. The only threat they face is Borrusia Dortmund, but the board knows the main reason behind their declined performance in bundesliga games and other top competition. They keep repeating the same mistakes they have made in the past decades, signing potential players and selling them after these players have fully developed into a standard form this season. There's alot to be achieved by Dortmund but they're not just ready, making unplanned activities and not serious in league games.
For Borussia Dortmund, there is no such goal as winning the league. If they put this goal on their priority list, they can be champions in a few years. Becoming a champion would bring great prestige to Borussia Dortmund. The closest they came to this in recent years was last year, when they lost the last match and missed the championship. I don't know when they can get close to that anymore.