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.
Rotating players is very risky, especially if they rarely do it. In the Champions League, their current position is safe and they have already ensured themselves to be in the round of 16, they can use this to rotate, because this match is not a crucial match.
I think they can do it, although they might want to maintain their victory. But in a situation like this, they also have to be smart in taking advantage of the situation because there is no more time for them to do it in the Champions League other than this match. In La Liga, it is also very risky to do it, except in other competitions in the domestic competition.