Do you think managers matters in a club? Ofcourse yes, appoints a confident manager and you will see the differences that follows but appoints a mediocre manager, you will also spot the changes in a particular club. Managers are essential because they're in full control of the game and clubs performance.
I used to feel that if a club had world-class players, any coach could manage that club and get good results because the players knew what to do on the pitch to win a game, but after I had seen how different coaches had failed to get good results in clubs that had world-class players, I agreed with you that getting a good manager is very important for a club that has world-class players to flourish and win matches easily.
PSG is one of the obvious example in the previous seasons when they have almost all world-class players in all positions including Messi and Neymar Jr in one team stil couldn't win the Champions League then with all the players they have. Now that they have less world-class players, they won the Champions League because they have a good coach.
World class coaches and world class players combined will always give good result that is why big teams don't usually sign small or average coaches with less experience, they only sign experienced coaches that will use their players well to win trophies for them.
Arsenal might improve next season because I think Arteta has learned a lot in his time at Arsenal, finishing second in three consecutive seasons is not a joke, and they are about to make a good signing of a striker that can score a lot of goals for them, that is why I said that they can win titles this season.
what is important when it comes to a team sport like football is getting a good team that should be playing as a unit and not necessarily big players who play more of an individual football which will not translate to the teams success and PSG players may not be that popular but they are playing as a unit now that before that is the difference that exists before and now