Unless you're Andy Serkis and you can make Netflix give your movie a theatrical release (
https://variety.com/2018/film/news/a...ut-1203022933/).
Maybe the best known example of this was
Fox releasing Idiocracy to fulfill contract requirements in Mike Judge's contract with no promotion or marketing just to say it had a theatrical release.
Contractual obligations might not be the actors - but director, producers, financiers, production companies, international distributors, etc. I highly doubt when any contracts were being drawn up, anyone thought this movie wasn't going to have a theatrical release. What actor's agent would want to forcing a studio to release an unwatchable mess of a movie? The concern would be financial with how back-end deals would be impacted. For lower budget films for small studios/production companies, some actors are doing films for practically scale and then hoping for a payoff (Blumhouse movies are a good example) when the movie does well at the box office. Some actors have gotten burned from those deals and now want more guarantees they aren't wasting their time. But that wouldn't be a movie on the scale of New Mutants.
Day-and-date VOD releases with limited theatrical runs allows essentially direct-to-video releases to actually be shown in theaters. So there really isn't the need to do what happened to Idiocracy again.
The only reason to not release New Mutants is because Marvel has plans to do something with the characters and doesn't want this version to taint future releases.