Denis believes Dune is the best movie he's ever made and that he spent 3 years designing it to be experienced in theaters. Which is exactly what people will be able to do if they want to once its released on October 1.
He states that AT&T has no love for cinema and its audience and that they're only doing this because they're in debt. And that while streaming is a positive addition, it cannot sustain the film industry.
He even goes so far to say with this action the studio might have just killed the Dune franchise because of piracy.
You can read his entirely over the top letter at the source