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.
it is very possible considering that there have not been many changes in the Real Madrid squad and with what Benzema cs has shown last season of course no one doubts that they will do the same or better for next season.
and it's true that Real Madrid's competitors will be even more fierce next season, some of the big La Liga teams have been improving for the better and of course with that to win the La Liga trophy will be even more fierce next season.