The arrival and departure of players is not a benchmark for a team to succeed and after all, Bayern Munich always has a better chance of winning the Bundesliga title because Bayern Munich is king. Regardless, Dortmund's unchanging way to win the title I think Dortmund will still be difficult to break Bayern Munich's dominance. Instead of expecting Dortmund to be Bayern Munich's rivals in the title race, I personally would prefer another team like Bayer Leverkusen, Leipzig or maybe Eintracht Frankfurt.
It will be difficult for Dortmund to win the league next season after selling out their players, although they brought in some fresh signings like Sébastien Haller and Karim Adeyemi from Ajax and RB Salzburg respectively, they will be able to finish in the top 3 but I doubt they can win the league next season. They will have to do better and bring in more talented players to be able to compete against Bayern Munich.