I might go along with this but the movie also features Wonder Woman who is every bit as powerful as Superman in this universe.
Compared to Cyborg, Flash — or even Aquaman and Batman — Wonder Woman didn’t have much of a character arc. Still, she had some nice scenes and had function within the narrative. So, yes, Superman might have filled a similar role. But if the choice is between Supes doing more-or-less what WW was
already doing (but gaining screentime) and the version of Supes we got in
ZSJL, then the latter (to me) sounds better.
In terms of Superman vs. WW power levels… I gather there’s an ongoing (friendly?) dispute within the comics canon as to who’s stronger. But in
ZSJL, it seems pretty obvious that Supes has the advantage. At one point, Diana says of Steppenwolf
“I've never seen a being this strong... maybe one.” The “one” she means is, of course, Superman. Then there’s the Heroes Park battle in which Supes dispatches the other JL members (including WW). And, finally, we’re given explicit exposition about the Mother Boxes. They remained dormant until Superman’s death. (
“It’s like they were afraid of him.”) I.e., the Boxes didn’t consider the JL (including WW) to be a significant threat. Now, you may not like or agree with Supes being at the top of the power hierarchy; but it can’t be denied that this is the “in-universe logic” that
ZSJL employs.
