It is undoubtedly true that even if Paris Saint-Germain wins the UEFA Champions League this season, it will be much more difficult for PSG to retain this title next season. If they can buy and retain players in the same formation, it's safe to say they could have a chance next season but right now it doesn't look like they will recruit any players. The two teams you mentioned have already taken important steps, with Manchester City already signing four players, even we can say that Real Madrid Football Club has also been eyeing various young players, one of which is an Argentine player. As far as I know, the young Mastatuno may join the Spanish club very soon.
The fact that teams buy new players only means they have problematic positions they need to strengthen. And if PSG are fine with their squad, have no injured players, and have decent squad depth, then they don’t have a big need to reinforce the team. Although, in principle, Guardiola probably thought the same at the start of last season. But the situation there was a bit different because they sold several very important players in the summer and weakened their bench. I don’t think PSG are at risk of that.