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 is common that if any key player (superstar goes away/transferred from the team) then it is common that there must be an effect on the team though few teams may overcome the absence faster and few may not be able to fill the absence of the player.
I don't see it your way, a superstar leaving a team usually has little or no impact on the team, this is because no serious club would let go of their star players when they know fully well that it will affect them, and there will always be replacement(s). But of course, except for the money which is not always common. We've seen many superstars leaving a club and the club did better than the time they were there, so it is about management and the luck shining on the team at that time.
However, it's all about love and how fans feel about the superstar which is peculiar to individuals since it's psychological. Some people follow a club due to the presence of their preferred players and many may also unfollow the club if their star players depart, that's the only consequence I see here.