I think it would be unfair to expect Bayer Leverkusen to repeat the performance they showed last season. Even teams with very large budgets and star players cannot achieve great success one after another.
Clubs can win titles season by season and we saw it with Lyon, PSG in Ligue 1, Juventus in Serie A, Bayern Munich in Bundesliga, Barcelona and Real Madrid in La Liga, Manchester United and Manchester City in Premier League. However, repeat unbeaten season is impossible and a latest example is Arsenal. After season 2003 - 2004 season, unbeatable, Arsenal in 20 latest years, did not repeat it again.
Expectation on Leverkusen is natural but realistically thinking, it's very hard for Leverkusen to have one more amazing season unbeatable like the last one. With their limited resources, even defending Bundesliga title against bigger challenge and pressure from Bayern Munich, is already a tough task.