Bayern Munich get stronger and stronger season after season.
As far as I can tell Dortmund has never been a good and big competitor's for Bayern Munich.
I don't think Bayern Munich is getting stronger but it is their competitors like Borussia Dortmund who are not improving themselves to be able to compete with Bayern Munich. If you look from season to season, actually the squad that Bayern Munich has does not have many significant changes, but they are able to maintain their game. But I don't see that in other Bundesliga teams.
I can't tell if Bayern is getting stronger in every season because Bayern has no competitors, no challenge, they still manage to retain their position every season. In the next season Bayern may be different from how it use be in previous season if Ronaldo is sign into the club and their target will be how to win the Champions League. Bayern Munich performance in the championship will tell if really the club is getting tougher.