Dortmund can still be considered successful in the Bundesliga in fact. Because for example they finished the last season at the second position in the standings. They didn't struggle to even reach this position at least. While there is a team like Bayern Munich it is still good to be able to finish a season right behind them. Of course Dortmund can fight better for the championship too but I don't see such future for them anytime soon. I think next season isn't going to be much different from the last one.
Dormund can only be successful in the Bundesliga if they stop selling out their best players, and consider building a strong squad which will be capable of resisting their rival Bayern. Otherwise, they will continue to end the League at the second play. Bayern Munich had won the Bundesliga for 10 consecutive seasons, and I think it high time Dormund should be able to compete with them. However, with the current state of Bayern squad strength. Is certain that they are the favourite in winning the Bundesliga title next season.
If Dortmund can win the Bundesliga trophy and end Bayern's dominance in the Bundesliga, it can be said to have achieved success.
So I agree with you, Dortmund should stop selling their star players and try to keep them. Because it will not be able to build a strong team
if Dortmund always loses its star players. Like this year Bayern lost Haaland, so I doubt that Karim Adeyemi, who was bought to replace
Haaland's role, can immediately adapt to the Dortmund game. So as usual Dortmund will likely return to playing inconsistently next season,
and will find it difficult to compete with Bayern. Moreover, Bayern have brought in several new players to strengthen their squad, and Bayern
still haven't lost Lewandowski. This means that the possibility of Bayern winning the Bundesliga trophy again next season is very high.
That's what I think the Bundesliga is a little boring, because the winner is very easy to predict.