Mittelstatd was always going to be a late bloomer, the kid was physically late to develop. He's always been a couple years behind the average physical development curve.
That said, at his current age, he's way ahead of where Hartman, Gaudeau, or Eriksson Ek were at that same age in terms of top-six center material.
Anyone labelling him a 4th line center likely doesn't.
It is fair to say that his value is not very high, and buffalo fans know that, that is why all but one guy are against trading him. (The one fan hates Mittelstadt, I think he slept with his girlfriend...)
But shitting on a young player that has shown as much upside as Mittelstadt has is unwarranted.