I don't think it was only because of Kane absence that Bayern Munich lost their match against Mainz. Bayern Munich played with a lot of injuries, they were without some key first team players like Manuel Neur, Alphonso Davies. Mainz played very well, they were able to take advantage of Bayern Munich’s weak defense. It was a step up from a disappointing outing against Wolfsburg in their previous match.
Bayarn Munich were unbeaten in the Bundesliga until the 13th match and thought that they might do better this season than last season but they could not maintain their consistent performance till the 14th match. They dropped points in 14th match against Mainz and losing this match is not good for them as another team in the league is not far behind them. If Bayern Munich perform like this in their next match, if Leverkusen get full points in their next match, they will be back level with Bayern Munich on points. I can partly blame the absence of Harry Kane as the reason for Bayarn Munich's defeat against Mainz because if this striker was there he would have scored and if he had scored the team would have won.
Bayern Munich declining is surprising me as well, because I thought this team would be the way they started from the beginning of the season, but I can see that their performance is not going as we expected. For Bayern Munich to lose to Mainz is too bad for them; being at the top of the league is not easy. They have to keep their consistency so they can be able to get the title this season.
Bayer Leverkusen is coming, and as you said, it is true: anything can happen. If Bayern Munich loses another match and Bayer Leverkusen wins any, they will take over, so I think Bayern Munich should do everything possible to continue winning their games.
However, Kane should not be a reason why Bayern Munich loses the match against Mainz, although I know he is one of their key players. I think some players they played with were injured, and that is why the result of the match is like this.