You might be wrong because City is suffering from bad performance and they are facing PSG in their next match. That match is their only opportunity thay will determine their qualification to the round of 16.
If they don't win PSG, they will go down the table and automatically out of the game because tbey will only have one last match to play that wouldn't change anything. I don't see them qualifying easily.
It's true as you say, Manchester City is struggling in the PL and CL and a win against PSG could change the whole situation for City and that's why I think they could win the game. It's almost Pep's last chance to turn the tide, if he fails here City will have their worst season since many years and also drop out of the CL, so the whole team will be very motivated.
Basically, both teams, PSG and Manchester City, of course, will do their best in their match later, because whoever loses in the match will make them probably have no chance to qualify for the play-off round. So, in this case, I think they will put aside their bad results in the Premier League first, and will fully focus on getting good results for the remaining two matches in the Champions League. So, even though PSG still looks fine, of course, in the decisive match for them, they will also have the ambition and target to successfully get three points. Assuming that both teams will be equally strong, then maybe the result in their match later will be a draw and thus, basically they will still have a chance to qualify for the play-off round.