I think it's fair, if slightly optimistic, to blame his slump on the rookie wall. The rookie wall is a real thing; there's no way these guys can really be prepared for 82, mentally or physically.
I also don't think it's fair to compare fields or shump to THJ. Unlike those two, THJ is a pure shooter and has a polished offensive game. Even if his D never comes around, he'll be a a quality NBA 6th man and that is basically the worst case I see. If he develops in other areas he could be a solid starting 2 guard by his 3rd year.
Nobody thought Fields was going to be a star; some of us just thought he'd be a great hustle and team player off the bench who could do a lot of little things. Basically we thought he'd be a nice role player, that was his ceiling. In the end it didn't seem like he had the mental toughness to be that kind of guy, and without a lot of talent, a lack of mental toughness will bury your career in a hurry. Speaking of which, a lot of Shump's problems are between the ears as well. The injury had to stunt his development, we can all agree on that. We expected more, because we thought he could at least be ok on O (he's not), while being a stud on D (he's not). You really don't know how durable someone is going to be or how solid they'll be mentally, until they play for a few years. But I think in the case of Hardaway, being who is and all, we're a lot less worried about his mental makeup.
Barring injury in a couple of years,
Worst case: Instant offense rotation guy or 6th man on a good team.
Best case: Above average starter, 3rd best player on a good team.
Most likely case: Serviceable starter at what has become a weak position in the league; not quite enough on D, but makes up for it with his O most of the time.