I think Rob has trade value, but I think that value is limited by a few things
(1) He's never been able to stay healthy for a significant stretch. He's dealt with nagging, off and on injuries of various kinds his whole career so far despite the fact that
(2) He's never played heavy minutes even when healthy.
Also
(3) Even at his best, Rob hasn't shown that he has shooting range or the ability to create his own shot. He's an elite finisher, shot blocker, rim runner, etc. This is relevant to his trade value because I think that's an archetype that teams value less than others in the modern NBA. Especially when you consider that despite his shot blocking Rob has never really been a great defensive anchor; he's just shown potential to be a rim protector given his athleticism, length, and shot blocking.
Given all of the above, I think there are really big question marks about Rob. Not just "can he be a star?" or "can he be a starting caliber center?" They're more fundamental than that .... "Can he be a consistent part of a rotation?" "Can he play significant minutes over the course of a full season?" "Can he play most of a regular season and also be healthy for a deep playoff run?" Related to that last one -- "Is he the kind of player who can stay on the floor in important minutes of close playoff games?"
There's a huge amount of variance in the kind of player Rob can become and what kind of value he can provide a team.