Rather than sacrifice Jake in the name of progressing on his journey of anger against the Man in Black, Roland now has himself a sidekick to teach the ways of the Gunslinger.
Thinking about the decision to keep Jake as a long-term character in The Dark Tower makes a lot of sense when you think of the context that Nikolaj Arcel has just laid down.
Though, to be fair, the Man in Black never presents him with the situation from The Gunslinger in which only one out of the two heroes can make it out alive and face him directly.
With the films and proposed TV component intended to serve as the final trip around the wheel for Roland and the Man in Black, our characters have to have learned something through their infinite battle.