Yes I will not be surprised when his performance next season declines. However, Benzema will help a lot, when he doesn't hit the maximum he will at least share his experience with his juniors, it's good to train the mentality of young players. Last season he also gave a lot of advice to his junior, Vini, so that Vinicius could impress in the team.
The performance of the Real Madrid team will not decline next season and players such as Benzema, Vinicius and several other players will still be at their best (except for injuries) and Real Madrid will always try to perform well from the start of next season because they are also aware that his opponents next season have started very strong.