With the signing of De Ligt and even though Bayern Munich lost Lewandowski. It feels like they are a stronger team now than they were 2 months ago. They needed better defensive players and they've now got De Ligt with a 5 year contract. In terms of attack, they've got Mane to run down the wings and create more chances.
Bayern Munich's defense and midfield are strong enough for next season. But with the loss of Lewandowski, I'd say Bayern Munich's attack will be a bit weaker. Sadio Mane cannot be considered as Lewandowski's replacement. In the last 3 seasons in the Bundesliga, Lewandowski played 94 matches and scored 110 goals. Sadio Mane doesn't seem to be able to score that many goals. So if Bayern Munich does not get a new striker, then they have to change their match strategy.