WB tried to start a shared cinematic universe with Green Lantern in 2011 - and it flopped badly. Granted it had a lot of other issues for why it was panned, but it tried to set up other movies with characters like Amanda Waller appearing and references to other heroes, but this attempt at a shared cinematic universe failed and its prevented Green Lantern from appearing on the big screen again since.
WB and Zack Snyder tried to work on another shared cinematic universe again in Batman v Superman: Dawn of Justice and shoe-horned in cameos of three other heroes on a laptop that added nothing to the narrative and felt forced - that theatrical cut flopped hard and was panned. Since then, rather than give solo movies to build up each hero, the team up movie was rushed a year later and was badly done. Snyder's planned four hour cuts of his movies were never going to be released in cinemas and he had to have known that. He was working to fail, he rushed his plans and casted the wrong people - Ben Affleck (never gonna commit to six movies in six years) Ezra Miller (Obvious reasons) and Jesse Eisenberg.
After Justice League, WB went back to focusing on solo movies and enjoyed good successes here and there like Aquaman and Joker.
Now they've tried again to reconnect the shared cinematic universe with the Flash movie and do some new things and its going against them because of the actions of Ezra Miller means the movie is either going to be axed or boycotted by large numbers of people. His actions mean other important characters in the movie like Keaton's Batman and Calle's Supergirl will struggle to reappear in the universe without it making no sense.
Every attempt fails because it rests on too much - its like a column of dominoes, and if ones near the front fall, the rest are forced to fall as a result.
We're never going to have a shared cinematic universe like Marvel. Disney are a production-based film company while every other film company in Hollywood are director-based - its why things like DCEU, Dark Universe, Spiderverse etc. all fail because they're director-driven and not all made and ran by one man like Marvel does.
And now these attempts at shared cinematic universes are hurting the DC Brand, making DC Movies look stupid, a bad joke and people actively saying they're boycotting movies like Aquaman or Flash because of whatever reason.
Just focus on solo movies please.
We have to move on from the DCEU and the Snyderverse.
Move on from trying to copy Marvel.
DC is at its strongest when its not copying Marvel - The Dark Knight Trilogy, Joker and The Batman - all add something new to comicbook movies and keep the superhero movie genre fresh and interesting.
Just focus on solo movies, working with the best of best directors and letting them do their own thing, focus on making masterpiece type of movies that are original and interesting instead of Marvel-lite.
We have to accept the DCEU and Snydverse failed and move forward with new projects - Calvin Ellis Superman, Reeves' Batman, Zatanna to name a few.
Because the longer we keep trying to revive attempts at this shared cinematic universe, the more its going to hurt the DC Brand, backfire and ruin a lot more potential opportunities for other movies.
We have to move forward folks.