Maybe I'm wrong but everything I've seen regarding this guy is that he's gonna project as a high-end second liner or solid first liner.
Obviously there is uncertainty regarding whether he achieves those levels but that's a great prospect to gamble on.
The issue also isn't whether Newhook is gonna be better than Kreider today when Newhook is 21 or 22, but rather where each player will be on their trajectory in 3 years. In 3 years Kreider will be 32 and he will likely be at the end of his time as a first liner and nearing the end of his time as a second liner. Meanwhile Newhook will be just beginning, and his upside (though not guaranteed) seems to be that maybe he's a 60 point player, not a 45-55 point player. But even if he is just a 45-55 point player, he'll just be starting the next ten years of his career doing that, at that point.
Or maybe Newhook doesn't get to that level -- but that's what a rebuild is all about. You are trading away guys who are aging and have value now for guys who you project to have value later.