There is a difference when a star player fits into the team. In football some days you'd need the intervention of a star to beat your opponent. It's great to work as a team and know the value of each other, but a star who is fine in striking can boost the tempo of the team player after scoring a goal or two. It's not just complete to be in a team with no special player. However a stat player can be worthless if the team play is not there. "A tree won't make a forest"
The presence of a star players can boost confidence all people in that team so they will feel that they have stronger than before and can beat all of the opponent. So when a star player leave the team, they feel that they don't have power like before and that can reduce their performance in the match while they will difficult to win. If a coach realize about this, he will use other ways to increase his player performance and convince all players that they still have power to defend and win the match. It is not just one man show but it is all people show in the team so they must unite and practice harder to increase their performance. With or without a star player, they can become strong and gives their best performance.
I agree that performance would decrease, as there will be less hype, less fans would cheer team, team mates would expect a more help during the game (from my experience, when there is a skilled player in the game, it makes other easier to play). If we speak about large sports clubs, then when the star leaves, game attendance and merch sales decreases, perhaps some sponsors might also leave. This all in total could lead to athletes salaries being decreased. And when someone is paid less than usual, not always he is giving all of himself during game.
Yes, many will decrease but all people in that team must anticipate it so they will not have feel sad. They can still rise from their sadness because they don't have a star player but that will not stop them to give their best performance and will be better than before. They will have a new method to practice and train themselves so they will have best performance when against the opponent. The presence of the star player gives them spirit to train themselves so they can get better thing in the future. That moment will be their memories that they can reminds but they will realize that they must have better skills and performance to beat all of their opponents.