I don’t think it’s particularly about the calibre or status of the actors, or even marvel vs dc, but more the parent companies. Disney has always been very strict and it’s not about protecting anything but their image and assets. Anyone who is a potential liability is quickly cast out of the kingdom. And yes, that can be good on occasion (*cough* gina carano *cough*), but they’re a corporation and do not have morals.
And look at the timelines, when he was at marvel-disney, the MCU was reaching the height of it’s popularity, it was setting new records, it was the hottest topic, hailed as “revolutionary” filmmaking. I have no doubt he was being a dick on set, he is who he is, but I also have no doubt that Disney leadership would’ve made it very clear to him that he was expendable. By the time the Avengers came out, the MCU had already released 5 films of some success, the Iron Man films particularly made a lot of money, and the franchise has a whole had already exceeded a billion dollars, which was a lot harder to do 10 years ago, and they very clearly would replace nearly anyone at the time. RDJ might’ve been hard to replace, since he was (and still is tbh) the face of the franchise, but they could get rid of anyone else. They replaced one of their own stars; Edward Norton was replaced with Mark Ruffalo, and the Hulk is a main character. Even if the film hadn’t done so poorly, if Edward Norton did not clash with anyone over the final cut of the film, he mostly likely would’ve been kept on as Bruce Banner.
Conversely, when he was at WB, not only was it by the time Disney had gained such a near monopoly over the film industry, but as a franchise, the DCEU had become stuck in a similar position to high fantasy/medieval fantasy shows after the garbage show game of thrones; no matter how different or unique they may try to be, they’re always going to be compared to the popular series. Of the four movies released prior to Whedon’s cut of Justice League, only Wonder Woman got acclaim from fans and critiques, and as with all companies, their bottom line was not met how they expected. Overall, all the movies were financially successful, but by 2016, it was not unreasonable for corporate execs to expect their superhero films to exceed a billion dollars, particularly with big crossover films like Batman vs Superman. Personally, I think the decision with keeping Whedon by the WB execs, despite the complaints wasn’t incompetency so much as it was a conscious decision and effort to repeat the success of the Avengers films specifically. It’s always about money, and while the dceu franchise and the films are more or less financially successful, they’re not particularly breaking or setting any new records. It’s not really talked about as much anymore, but there was fan and some critical backlash to Age of Ultron, it was a crappy movie and there was classic Joss Whedon sexism in it, but despite that the movie still made 1.4 billion dollars, and that’s what the execs wanted. They had sunk 300mil into making the film and hoped that by repeating what seemed to be a successful recipe, they could make it back and then some, even if the movie was crap, even if the actors and crew suffered. Whatever else they may say and however they try to cover their asses now, they real reason Whedon won’t be directing anymore justice league movies is because he failed. The movie was a box office bomb, when as a big team up movie, it should’ve exceeded a billion.