You're right but the problem with the bundesliga is that other teams are not serious about building their teams
How do you know they are not serious? They may be serious but they have no enough money to build their teams. You can't expect every team easily buying new players, they may have limited funds.
so as to challenge for the bundesliga title the only team I see challenging bayern munich for the title is Leverkusen that is if they can strengthen their team by bringing in quality players especially at the attack because they attack was poor and even the defense line up, apart from Leverkusen I don't see any team that can challenge bayern munich
IMO, Dortmund mostly becomes the biggest rival of Bayern Munich, they often finished as a runner up. Leverkusen only became stronger in the last 2 seasons, they rarely finished in the top 3 in the past. Nowadays, I think we have 5 strong teams to be the potential competitors of Bayern Munich in the next season (Leverkusen, Dortmund, Leipzig, Frankfurt, Freiburg). These teams look improving their squad gradually in the last few seasons. I'm sure they have a chance to be stronger teams in the future.