Let's put it this way, Barnes on the Clippers last year doesn't get them a championship. He's an average defender. Similar low usage wings (Middleton and Crowder) have outperformed him, and when you look at other advanced metrics that are less usage dependent he sucks there too.
I understand why he's going to get overpaid this summer. The potential is clearly there and he has shown flashes but I think it's just as likely that the Warriors system is propping him up as it is that the system is holding him back. Also 24 isn't that young, most players have reached or are pretty close to their peak by age 24.
If Barnes ends up being Gay, Parsons, or even Middleton then we will look back in three years and say "yeah he was overpaid but that was a crazy summer." If he ends up just being Harrison Barnes then the 76ers will be taking whatever forward Duke has to offer that year in the top 3.