Unless Porzingis actively wants out, the Knicks have no incentive to move him for a draft pick. He's only 21 years old. If they can move Carmelo and try to rebuild through the draft they don't have to move him because he's still on the timeline. He's a young player with superstar potential. Trading him a pick, who are always question marks to some degree, I just don't see happening.
But I do still think that a deal built around Isaiah and Horford for Porzingis could make a lot of sense for both teams. But only if the Celtics land a top 2 pick in the draft (and, actually, that kind of short term step back might be unpalatable if the C's make it to the ECF and make some noise there). And even that might not be enough for Porzingis, even if he does demand out.