He's too small to play as an NBA SF. What's the point in trotting out someone who's like 6'4" or 6'5" and asking him to defend guys who are 6'8" or 6'9" every night?
I don't think Okoro is an offensive liability--he started to come on as last year progressed, and I expect that to continue. If he never figures it out, fine--you have another defective/limited player and you look to get someone who can fill that role better.
I think you're exaggerating the height disadvantage here. Okoro's the same size as Moody, Keldon Johnson, Josh Green, Joe Harris, KCP, Hardaway Jr, Crowder, and many other guys who regularly play multiple wing positions. And he's as talented defensively as any of them. It's not a big ask for him to play a significant chunk of his minutes at SF. Much bigger ask for him to be the 2nd option offensively, which is what he'd be at SG unless we spend big bucks of a free agent SF.