People should stop feeling the need to compare Giannis and Kelly.
It's probably true that odds are Giannis will have a better career at this point, but by all indications Kelly is still a good player and a fine pickup at #13.
Just because Giannis was available at the time doesn't make Kelly a bad pickup. Giannis was a high-risk, high-return play which has just begun to look like it might be pay off eventually.
Kelly was a medium-risk, medium-return play that looks like it might be paying off as well. It's just that the payoff is a bit smaller, but that's because it was lower risk to begin with.
They are both fine picks.