Bayern did not have the choice to play anything else because of a small squad and injuries. They even had to play players that had problems the day before. And - as Demichelis' U23 is fighting for staying in the 3rd league (what is very important for the club's talent development and we needed 8 years to get back from 4th into 3rd and there is 5 fourth leagues that fight for 3 spots going up) Bayern could not just take the best under 23s.
Bayern started with a Kimmich/Müller double pivot with Dantas as no. 10 who is great on the ball but has the physics of a U15 player. He actually did pretty fine - better than I have seen him in the matches he did for the U23.
Flick said after the match that he wanted to give Kimmich and Müller atleast a 20 minute rest at the end of the match - but as other players had problems he could not. Kimmich ran 12.8 km Saturday against Leipzig, probably not less atleast not less intensive against PSG and 13.1 km yesterday. We only have 6 days between both PSG matches - not 8. And play the teams ranked 2nd and 3rd in the league away directly before and after the PSG matches.