Superman will outlive DC comics
found yer problem rightchere
If Batman and Wonder Woman can make money on their own, there is definitely a problem if Superman cant when he use to be of equal importance as Batman back in the day. If they have other heroes, like say Aquaman, who are more popular they are going to want to do more with them than Superman.
What we have been seeing is DC trying to make more than just Batman big over the last decade. And there have been things like Green Lantern that greatly damaged the character and interest from DC doing much them, while other efforts (WW, Aquman, Shazam) went well and those heroes are rising in prominence. Superman is just the toughest one to crack because he is well known but the masses
just cant care about him in a way that makes him viable in the hero film market
found yer problem rightchere