They would be smart to do so too, as he’s a major attracting factor for fans of the production right now, and many people keep coming back to “Supernatural” just because of Castiel. It used to be mostly about the two brothers, but now the focus has shifted somewhat.
There are reports that this is what the producers wanted to happen from the very beginning with this character, and that the fact that some fans were more skeptical towards him was an unintended effect. Plus, in all fairness, those fans weren’t actually that many in the context of the show’s whole audience.
It would be interesting to see what’s planned for Castiel for the next episodes, and how the possible conflict between the two brothers is going to play into his own story as well.
It might turn out that the show is leading up to a more dramatic change that will alter its landscape for good, but we would have probably heard a few rumors about that already if it was true, especially given how actively fans of this show are looking out for new information about it. It’s very hard to believe that this kind of rumor would have slipped under the radar, although there is so much noise in the field of rumors for this production that it’s not impossible.
While Cas (and Misha Collins) both have a lot of fans – and deservedly so – I think it might be an overestimate to say that the show is no longer about the two brothers, or that the producers intend it to be so. Anyone who has followed the chatter from the cons, particularly SDCC, knows that the producers’ and the two leads’ major comment for months has been “It’s about the brothers. This season is all about the brothers. We’re going back to it being all about the brothers.”