I've actually been thinking a lot lately on how I'd like the show to end and I completely agree with you on the last shot being, if not all three of them, just sam and dean. I live and breathe destiel, but I get that the foundation the show was built upon is sam and dean. It would feel wrong to leave sam out.
So wrong. I think that everyone would be upset if that happened. The show is, at its core, about Sam and Dean. Cas is important, and depending on what happens in s9 this may change, but...he's important in the context of Dean's character. I would love to see Cas get his own story arc and become a main character in his own right, like we came so close to doing in 6.20. But as it stands right now, the show is still about Sam and Dean, with Cas as a major player but still a side character. And leaving out Sam in the final shot would be an insult to the foundation of the show.
And this is coming from a girl for whom Sam is not the emotional investment in the show. Understand that I love Sam. A lot. Jared is a fucking phenomenal actor and brings so many dimensions to Sam that it makes me want to weep. BUT...for whatever reason, when I sit down to watch a random episode, Sam's story arcs don't make me make inhuman noises at the television.
I'm a Dean girl. I'm a Cas girl. I'm a DeanCas girl. And even I think it would be an injustice to Sam for the last shot in the series to leave him out in favour of Cas.