The fact the movie was bad and looked bad does. Again: You can blame the opening weekend on Ezra, the DCEU, whatever. But the quality of the movie and the visuals, which Muschietti was directly in charge off (and let's not go over this fairytale that "Oh poor Muschietti he suffered from pressure by higher ups and they meddled with his artistic vision", BS. Muschietti was boasting about how the film was his in the entire press tour. He's given zero indication he was blindsided or done wrong in any way) you can absolutely blame on him and its subsequent effect with the B Cinemascore and the widespread rejection by audiences that lead it to have the absolute worst legs of any DC film in existence.
I'm kinda tired of this trend of constantly defending directors whenever a bad movie turns out bad—sometimes directors just do **** jobs. No one spent more time working on The Flash than Muschietti, and he was still the one that delivered the worst visuals of any blockbuster in the last few years. He was still the one that gave the movie its schizophrenic tone which you can blame on him since it was the same in IT Chapter 2.
The same exact reasons he was picked is the same exact reasons he should and will get the boot. He's replaceable, he has nothing to offer, he's a journeyman with no vision or creative instincts. You can find 15 other guys like him that'd do a better job. Both IT Chapter 2 and The Flash have proven that he's a studio puppet, yes, but he's an extremely bad and unreliable one. No company will want to take another 200 million dollar gamble on him.