Barnes is a good footballer but at Super level he's a better 12 than 10 (he might even be a better 15). He simply does not have the right style of play to be a championship 10. He's much better there in tests where there is more talent outside him and when a tighter, more conservative style of rugby is played.
Here are the 10s who have won in Super Rugby: Carlos Spencer, Andrew Mehrtens, Bernie Larkham, Dan Carter, Quade Cooper, Morne Steyn.
With the exception of Steyn, those guys are all world class 10s and Steyn is particularly suited to the Bulls style (and is a World Cup champion).
Whether Foley would ever be in a similar conversation we don't know. But at this point surely we do know that Barnes won't ever be.