He is simply not NHL ready. Not sure what we're doing here. I expect they are really banking he is a fast learner and etc, but this can't be what's best for his development in the long run....
A "ready" NHL prospect is one who will not be in the pressbox, who does not need extreme sheltering, and not only has more skill than his internal competition, but demonstrates it being actually effective more often than their skills.
You may think AHL development is better than I do, but for you to say that Slaf is simply not ready (as if it's obvious) flies in the face of an entire professional organization's assessment.
As for whether it is best for his development, it has been best for high draft picks for multiple teams, multiple years, multiple players. It is NOT "obvious" that he cannot follow in their path.
If he can't produce, and four out of Pezzetta, Pitlick, Dadonov, Hoffman, Drouin, Armia push him out of a job, that is fine, send him down.
But right now, he has CLEARLY shown to his coaches and GM that he is ready in their eyes - to START developing at the NHL level on a 3rd or maybe 4th line.