Sure, you can have believeable relationships on screen while not having it off screen.... but if you are doing an ongoing franschise, or heck, ANYTHING long term... MANY effective teams have a kinship way beyond just the job.
I will give you another example: it is well known that on the 1966-68
Batman TV series, Neil Hamilton (Commissioner Gordon) did not get along with Stafford Repp (Chief O'Hara) off-set due to the former disliking Repp's exaggerated performances, but once the cameras were rolling, they played their parts with no one questioning if the characters' particular relationship worked. They were actors, and once on set, they knew what they had to do.
With the DC Film Universe, fully 1/2 of the Justice League is basically out of it... Ben Affleck is dealing with his personal issues (which led to the Reeves one rather than an Affleck-directed one), Fisher has burned bridges so that the Cyborg solo movie will never happen (or basically happned within the 4 hour movie), and even after the Flash movie... will Miller be able to stay?
In Affleck's case, he's in the Flash movie, and has stated it was supposed to be his last appearance, but the grain-of-salt-required rumor mill suggests he
might be interested in another turn as Batman. Who knows.
Fisher was the victim in his case, and stood his justified ground, so in the wake of his being removed from the WB productions, its WB that has the appearance of mistreating the injured party. ...and you're right, Cyborg did receive his spotlight in the
Snyder Cut, so at least for Fisher, his work with the character / good screen time is not lost or in the
"what if?" category.
We're not quite sure what is going on with Cavill (who was well cast, but poorly written)... Momoa is busy with many projects, but i am sure is fine with being Aquaman independent of Justice League (though happy to do a Peacemaker cameo) , as is Gal Gadot.
Cavill's Superman story was handled well, particularly for a film set in this period of time.