Supernatural showrunners credit Misha Collins specifically as Castiel, and the angels' storyline generally, as contributing to the show's rejuvenation and as part of its longevity.
Erik Kripke (at min 0.13):
You know, and I get this question a lot. I mean, here's the reality, and I'm always accused of being, probably too honest. You start these things with like a cocktail napkin sketch of the five-year plan. I mean, the five-year plan is a couple of pages. It's not much.
And it was always driven by the demon side of it. So, yes, the progression of the demon ladder ending with Sam inadvertently going dark side, bringing on the apocalypse and having to deal with the apocalypse and kind of moving up the ladder from Yellow Eyes to Lillith to Lucifer. That's the plan we've been on.
You know, the angels, you can go back and find interviews that I gave as far as - there will never be angels on the show over my dead body - hur, hur.
And, you know, obviously it's probably one of the coolest parts of the show. And that came from us, I think in the writers room, realizing that, like, we had a really cool, evil empire, but we didn't have, outside of the few cool hunters we had, we didn't really have the rebellion. Like, we didn't have, you know, you couldn't have these awesome off-camera battles that sort of lent a real epic feel to the show.
And that we felt there was a missing puzzle piece that kind of evolved through as we went along. And that's where really the angels came from.
And thank God, they did because totally rejuvenated season four in no small part to this guy (gestures to Misha).
And so, the angels quite frankly evolved; that part did evolve as the process. But the part of building up and facing down the devil has, that part at least, that thumbnail has kind of almost been around since the beginning.
Source: TV Guide 2014 SDCC edition
Castiel helped send the series in a different direction, introducing the notion of Heaven. Beyond the [appeal of the Winchester] boys and their chemistry, he's the biggest reason that this show has lasted.
Source: Supernatural Homecoming Exploring Episode 300
Andrew Dabb (at 3.00 mark):
Cas's introduction in season four, the introduction of angels generally, is probably the reason the show has continued and lived as long as it has.