I don't know why PSG didn't bring in some defenders during the transfer window, but there seems to be a factor of importance at play in their transfer policy so far. They also failed to improve their strength in the midfield position, I mean PSG also doesn't have better quality midfielders. So PSG will not be able to compete with other strong teams if their squad depth is only good in the attack line, while they fail to find quality midfielders and defenders.
This off-season, PSG did not strengthen the team, maybe the reason is that after many years of spending a lot of money, some kind of disappointment followed and they decided to take a break from this endless waste of money. But in this case, they shouldn’t count on results, their attack was also significantly weakened due to the departure of Messi and Neymar. Mbappe alone will not be able to solve all the team’s problems.