Superstars always color matches and help their team win games in times of difficult games. And when they leave there is always something to miss about them, the fan love, fans switching clubs, and most important the team losing top notch performance. Recently, Marquinhos made a statement that he
misses the days of Mbappe in PSG. He's sure that their draw wouldn't have happened in the watch of a no "9" like him. Also PSG fans would feel the absence of Messi and Neymar's brilliant skills. On the other hand, Manchester United also should be missing those moments of CR7.
What is no longer there in your club after a certain key player was transferred?
it's a normal thing for fans to feel less connected to thier club when a certain player leaves the club , it's usually deficult to enjoy watching those games without those key players but overtime, we still get to find a replacement in the team that brings back the love you have for the team.
No player will stay in a team forever. You can only play for a particular numbers of seasons after which you get to exit and join another club. Observe that most real Madrid fans that supported the club because of Ronaldo are all supporting Al Nasir and if you also check the fan base of inter Miami, you will find a lot of messi supporters in those club. If iether Ronaldo, Messi or the both of them leaves inter Miami and Al Nasir, I can bet that the name of the club won't receive the kind of recognition it is currently getting. For the teams overall performance after a star player has left, the effect isn't always long term as long as the rest of the players are ready to play heartily for the club. Mbape left PSG and they are still doing well without him. Chelsea has done away with all the players that fans related well with in the early days of the club. That era has long gone and gradually fans have started getting used to the faces you will see in the Chelsea games. It's no longer the era of Lampard, drogba, Micheal obi or rudiger. It's the era of Cole Palmer and Jackson and they are all still fighting for the jersey.
As a core fan, players might come and go and in some seasons, they might underperform but the love will always remain intact.