Also, according to
Variety, Disney is contractually obligated to theatrically release all finished movies, or films in post-production, that were part of Fox’s schedule at the time of the the acquisition.
Seemingly the only way for Disney to move The New Mutants to streaming at this point, per
Collider, is for Josh Boone and the cast to "amend their contracts and sign off on a new deal" and for Disney to also somehow renegotiate Fox's HBO contract. Is it achievable? Probably. Is it worth the hassle? Well, that's for Disney to decide.