Austin, I read that interview (thoughtfully stripped of spoilers), and I didn't get quite that impression, that the leads are negative about the show--just about going past the five-year mark.
They signed on for five. They were told they'd be filming in and around LA, and they had no idea that the shooting schedule for the leads was going to be so unremitting and exhausting. Jensen's gone from 26 to 31--he'll be 32 or 33 when Season five wraps. Jared will be 28 or 29. If they're going to do name-making solid work, this is the time for them to do it. They can't shoehorn it in with the paltry few things and the type of things filming during their three-month hiatus. They need some flexibility and time to choose good vehicles and make decent mainstream movies.
They both went from believing they'd be working in the heart of their industry, where they'd get early news of projects and jobs and possibilities, to an outpost in another country, and I understand them feeling it was a little bait and switch. I think they've been remarkably good natured about it, but I certainly understand them wanting that situation to end when the contract does.
Kripke said from year one that he had a five-year arc and he hoped the network would let him tell that story. He's said more recently that after five, he's out of story. He's willing to set up a few monsters and possible storylines and directions, but after five, he's out. And I really don't think either of the leads would want to stay, especially having lost Kim Manners on-site, if their head creator and writer was no longer connected with the show.
The Brinks truck remark was in response to the interviewer suggesting that Dawn Ostroff break out her checkbook--that actors traditionally bicker over money when it's time to renew contracts. It's expected that actors will push for huge increases in order to sign for another season or two. I just took it at face value that Dawn's checkbook isn't big enough--money's not the object this time. Jensen wants to finish up Supernatural the way Kripke envisioned it, tie it off with a bow and walk away proud of what they've done.
I completely understand Jared's exhaustion with the work, PR, work, PR self-perpetuating circle, too. I don't doubt that he's completely dedicated to making Season five a fantastic season. But he's ready to do something else afterward.
It's ironic that the show has picked up so many new viewers and is now seen as a success for the network. I think the feeling among the newbies is "Shiny new thing! More!", and this is what the J's are getting in interviews and cons. It was end of season when that article was done, and they were probably both slap wore out, as they usually are at season's end. But I also think they're past being cagey about extending the show another season or two. They've said since the beginning that they don't want to fade off like the X-Files, that Kripke had a plan, and they hoped to get to tell that story. They've never made a secret of the fact that they were all in for five, and now that they're being pressed about it, they're repeating themselves and being more firm about it.
I'm going to be sad there's no new SPN on my screen, but I do approve of the five-year plan. I think the whole cast and crew are going to hit it hard and do fantastic work *because* it's the last time, the last chance, and because they love it and want it to go out great. At least I hope so.
ETA: Or, you know, what everybody else said much more succinctly while I was composing my opus.