I mean technically speaking all teams have "bought" players, how many teams won UCL by raising youth players from their own system? It just that some teams have more players that played together, and some teams have less. City for example won it this year and looking at them they had players who have been there for only 2-3 seasons so far on their starting team as well, even just their rookie year like Haaland for example, so it is not a must.
It doesn't guarantee that players been together for many years neither, sometimes it helps, but that doesn't mean you will win. At the end of the day what matters is talent of the players and how much they fit the system of the manager who has to be talented as well. If you have that, you have a good chance to win.
PSG is a big team currently the best in Ligue 1 for years and are financially capable of getting any top players but what they need is a good coach like Pep Guardiola that would build a very strong squad and instill a winning mentality and probably lead them to victory in the Champions league which they're desperately in need of, as for players they brought in good players to replace the departed ones and so far they've been impressive in the Champions league and has to maintain that form to be able to succeed in the tournament.
Mbappe is the most reliable player for the team and is very important for them due to his goal scoring ability and they'll need him to be in form so as to help them try achieve the Champions league this season, but I feel Mbappe might seize to be a player of PSG by next season.