TEn years isn't an absolute sure thing. Its just very likely.If the CW manages to have success with every single new pilot next year, and if SPN slips more then usual it would very likely get axed.
Its unlikely those events (especially the first) would happen. But its nota sure thing.
What alway kills me is every year I here people say I thought it was its last season (going back to season three even, which after it was taken fully off the air for a month I could at least understand), but after that. Its ratings never had it anywhere near being in jeopardy. Getting early renewals each and every year, since season three (though that early renewal has never been so early for the CW).