I'm pretty torn on the issue.
The problem is that there are other excellent female actors out there who could have played the part of Buffy. The odds of having an actor as good as SMG for the part were probably low, but it is possible that Joss would have found another suitable Buffy. On the other hand, if not for Joss, SMG would more than likely have been successful in some other show - perhaps less successful, perhaps more. But that show wouldn't have been BtVS.
So without SMG, there is a possibility BtVS would have been as great. Without Joss, there would be no BtVS, but SMG might have made another show great. If we were asked to choose which of the two is more important to the TV industry in general, it'd be a much tougher choice, but I think we'd have to concede Joss's impoirtance to BtVS, seeing as how he created BtVS.