Far more relevant that whether Folau could play 13 (he probably could given some time from what we've seen so far) is who realistically backs him up at 15. There are plenty of players lurking around who have played or are playing 15, but I wouldn't say anyone is really a standout if Folau wasn't there. I suspect we'll end up with a "utility" player being the 15 if Folau is injured. And a big, big step down in class.
I can't see Folau being started at 13, simply because any coach would probably want both him and Kuridrani on the field at the same time, and currently (and also looking at RWC timeframe) I can't see either being moved.
I have no doubt that theoretically, Folau could be a good 13, and Kuridrani could probably be a good 12 if anyone wanted to explore that possibility. Down the track. That's a flight of fancy for later.
As Brumbieman says quite rightly, we need consistency for now, and experimentation can be done later if it needs to be. Kuridrani should rightly be well in front in any discussion - he has plenty of classy displays on the board. So too Folau at 15.