That happens to every club, nobody stays at the top for long
Except for Real Madrid.
Chelsea always peaks for like 3-4 years, then they have a couple of down seasons, club splashes cash, and they're back again. It's been a thing since Mourinho came in the mid-200s. They've been a rollercoaster ride for awhile now.
Bayern has been the same for about 3 decades now. 4-5 years of dominance, they get complacent, have 1-2 down seasons (which doesn't mean much as they can still win the league when theyre shit in europe), some clubs start to catch up and challenge them, they freak out, get a new coach and/or new key signings, they're revitalized and back to completely dominating on all fronts.
The only club that has truly gone past their cycle of dominance in the past decade is Manchester United and the Italian clubs. Most elite clubs tend to not actually struggle on the long run.
Though it remains to be seen whether Barcelona's period of downturn will be extremely short term or an actual cycle mediocrity that'll last the better part of a decade. This will be our 3rd mediocre season in terms of results, and like our 4th with a mediocre squad.