Admittedly, last season Dortmund's mentality could only reach runners-up in the Bundesliga, even though the title was right before their eyes, but they failed in the last match against Mainz. I agree with you, this is because many Dortmund players are sold just to get money and not a few Dortmund players are sold instead to opposing teams like Bayer Munchen. So do not be surprised if Bayer Munich's domination in the Bundesliga is difficult to be broken by other clubs, including Dortmund.
In the last season, Dortmund did not prepare to be the champion. They were not ready to compete with Bayern Munich after selling Haaland to Manchester City. When they suddenly got big chance because Bayern Munich suddenly got a worse season, they were not ready to convert it to a title. Many other clubs got a same chance like Dortmund but failed to beat Bayern Munich too.
Dortmund lost direction last season and Bayern Munich became the strongest with significant developments in their play.
When Haaland went and played for Manchester City he became a greater player and even managed to bring Manchester City to win 3 different leagues in one season.
But for next season I see that Dortmund have had pretty good preparations and now is the time to see Dortmund compete again with Bayern Munich.