I think Dortmund realized themselves very late otherwise they would have replaced their manager for long when they started losing matches back to back by now they would have been comfortable in the league without waiting for other teams to drop points before they can have the opportunity of winning their own games to qualify for the champions League.
Personally, I think they shouldn't qualify for champions League next season so that they will be motivated to improve their performance as well as focus more on the league matches so that they will be able to fight for the league title next season.
You bring up good point about when Dortmund changed their coach. It is easy to look back and think that if they had done it sooner when they started losing much they might be in better spot now. They would not have to hope for other teams to lose so they could get chance to qualify for Champions League.
It is also interesting that maybe it would be better if they did not qualify for Champions League next season. You think that may make them want to play better and focus more on league games. That way they may have better chance of winning league title next time. Sometimes when things do not go well it gives chance to people to work harder. It would be different for Dortmund if they were not in Champions League. They could put all their energy into games in their own country. We do noy know if that would make them win league but it is good idea about how they could get more focused.