I agree with you that Hansi Flick should rotate players in a safe match like this so that the main players can maintain their fitness, especially with the increasingly tight competition in La Liga and the big challenge in the Champions League that awaits. Giving opportunities to young players or reserves can also be a good step to maintain squad depth so that the team's balance is maintained.
Rotation might be possible in cup games, or even against Atalanta today, although Flick might want to keep his place in the table to gain an advantage against his next opponent in the knockout stages. In La Liga, Barcelona don't have the luxury of rotating their young players because they need to think about how to get back into contention against Real, who have already extended their lead considerably.