I don't know if what I'm about to say is an absolute truth (and I'm not sure that's well articulate enough), but I think we have to bear in mind that the golden age of superhero movies has now lasted almost 15 years. We can talk about a certain fatigue setting in, but I think there's also a generational parameter to take into account.
Taking the MCU as a symbol of all this, I've always considered that part of its enormous success was also due to the fact that many kids born in the 2000s grew up with this saga. These films accompanied them and fed their imagination for an entire decade, practically an entire childhood.
In my opinion, there are two reasons for the drop in attendance at these films right now. The first is that this saga (and the genre as a whole, with its "extended-universe" ideal) came to a certain conclusion with Endgame and saw the departure of several of its most famous figures. The second is that this "ending" coincides with the moment when most of its audience is entering adulthood and perhaps no longer has the same interest or time to devote to it (because of work, family, life). Add to this the objective decline in the quality of these films and the difficulty of finding a new direction, and you've got the perfect recipe for making it easy to split with part of the audience and failing to renew it.
Of course, there will always be fans to follow everything, whatever their age, but it seems to me that today, in the usual age range of moviegoers, i.e. 8-25 age group, interest has naturally shifted to other things, particularly manga and anime. In any case, if my environment is anything to go by, we now have teenagers and even young adults who, for example, have never seen Nolan's Batman trilogy, Iron Man and so on. Because they were just too young to catch it while it was developping or because, quite simply, they've their own other things...
Of course, that doesn't mean they can't love CBM. But the challenge for DCU will be to reach these people, because as I said, I don't think the target audience is as “dedicated to the cause” as it used to be. I can hear the argument that the pandemic slowed things down, but that was four years ago, enough time to see trends appear or fade. And I'm not even talking about changes in the way movies are consumed, which is a whole other, but impactful, subject.
Of course I can be completely wrong with this “analysis” but my point is that in any case, realistically, DC and Warner should set themselves targets for figures in the range of what a good CBM could do around 2010. This Superman will be released in a very different context than recent CBM and I'm not sure Gunn is bulletproof (no pun intended).