I think Oubre is still likely to be at least as successful as Stanley Johnson, simply because he's prototypical for the wing positions. There aren't many 6'7" or 6'8" 245-lb players in the league having an impact as starters. Basically, you've got Millsap and Draymond Green. Well, and Lebron James.
Motivation has a lot to do with how successful you are in the NBA. If you're not motivated, there are 100's of other players out there hungrier than you, wanting to take your place.
And apparently this is the case with Otto Porter. As this revealing piece of interview by Paul Pierce:
"Otto is another one who just doesn't understand how good he is,'' Pierce said. "He can shoot, he's a slasher, he can defend, I'm just not sure how badly he wants it every day.
"That kid just needs to get mad. If he came to practice ticked off and to the games ticked off, he'd be fine. But it's hard to get Otto mad. I should punch him one day just to get him riled up.''
Oubre seems a lot like Porter on paper. Long reach for the position. Nice stroke. Good to great shooter. But without motivation, he's not going anywhere fast. May be he is yet to reach his potential, who knows? But as it stands he needs to pick it up.