I don't know about that. Seasons 6 and 7's story arcs both had their share of problems, but there's another issue that's been festering lately: they're running out of things to do.
By this point they've done episodes about on just about every major myth or urban legend. They've been having to scrape the bottom of the barrel for monster-of-the-week episodes, and the leviathans were pretty much just made from scratch (you could say Lovecraft's work inspired them, but they neither look nor act like the cosmic horrors he's famous for). People like for the monsters to be creatures they've heard of, but at the same time, there's only so many vampire episodes you can make before viewers get sick of them. Worse, the characters themselves are getting tired -they need to grow and change in order to remain interesting, but by this point Sam and Dean have been through so much there's not much that can be done to shake them anymore. Just look at Dean's arc (or lack thereof) this season.
Even if they can come up with a compelling plot for season 8 and beyond, I'm not sure they'll be able to keep people interested in the filler episodes that make up 60-70% of the season. I love this show, and I plan to keep watching it for as long as I enjoy it, but even the best series can only go on for so long before they start to get stale.
