Unless the CW goes out of business, SN will be back for a seventh season. Its outperforming most of the CW's other scripted shows (With only Vampire Dairies and Smallville outperforming it. The leads are signed for a seventh season. Its performing very well on cable stripping (in fact its 10am showings are doing just slightly less then its reruns on the CW (which is amazing for 10am and the fact that cable reaches viewer homes), its DVD sales are still the strongest of any property of the CW. And while DVR results really don't bring in much in revenue, they do show viewer interest, and the numbers of people who watch within 7 days (as of the data up to date to two weeks ago) is equal to the number of people who watched with in 7 days to the same week last year.
With shows like Nikita doing worse across the board with the largest leading the CW can provide. With Sn on a Friday beating it in adults 18-49, adults 18-34 (and in both genders), Nikita is more of a question mark then Supernatural.