I see no way to say that Harry Kane's performance is poor. Harry Kane performed well even though Munich did not win the title last season. And Kane's performance this season is also very good. Kane has 5 goals and 4 assists in 5 Bundesliga games. He also scored four goals in the Champions League. So I don't think there is any weakness in Harry Kane's performance. Bayern Munich won't benefit much from adding Harry Kane's replacement to the squad.
I wouldn't say Bayern Munich lost against Aston Villa with a poor performance. We saw the dominance of Bayern Munich throughout the match. But even then, Munich lost 1-0 in the end. This is Munich's misfortune.
No striker player in the European top five league is better than Harry Kane this season, Harry Kane had a good performance both last season and this season, Bayern Munich was defeat against Aston Villa doesn't mean that Harry Kane is not a good player, Bayern Munich play very well in the match but Aston villa play a defensive game to win the match, we can't be expecting a team to be winning games every time, we should remember that we are talking about game and the game is either winning or lose from the both teams that play against each other.
I know some people will still be doubting that Bayern Munich can not win the Bundesliga league this season again just because of the defeat from the champion League but for me I'm very sure that Bayern Munich can win the league title this season, Harry Kane is trying his best to make sure he won a league trophy and I believe that this season will be a good season for him.