Just because Kripke had a 5 year arc (one that he changed dramatically as time went by) doesn't mean that the show was supposed to end. THat isn't a call a creator can make.
And yes SPN will most likely make 10 years (the two leads signed contract extension to cover seasons 9 and 10 late last year). Unless somehow the CW manages to find shows that outperform it, and in large numbers, or if all of a sudden SPN drops dramatically, next season). Out of the 7 years on the CW, only Vampire Dairies has managed to out perfom SPN. Even Arrow which just 11 episodes was scoring 30% stronger then SPN has dropped to tying SPN for its last two weeks. Arrow still has good viewer numbers, but unfortunately viewers over 50 plus really have almost no impact on a shows revenue. And thats were the primary difference between SPN and Arrow has been the last month.
As for the CW, while its a joke of a network, it still plays like all other networks. Meaning it isn't going to cancel its 2nd or 3rd strongest performer just because its been around for a while.
The one exception for this is if, your show became so expensive to produce that it isn't worth it. Or if the cast no longer wishes to stay. Well the cost to produce SPN has always been on the cheaper side, and with only two cast members and very few producers from the beginning, its cost haven't gone up dramatically.