If Rosberg had suffered a string of mysterious mechanical failures all season, then maybe there'd be something to go on.
That said, parts fail from time to time, and it happens to anyone and everyone.
In my opinion, Nico has had every opportunity to score as many points as Hamilton, he just hasn't capatilized in his opportunities, and in a couple cases, drove like a bit of a push-over. He has no one to look at but himself.
And unless he wins the WDC next season, I can't see him going anywhere but staying at Merc, or stepping down to a mid pack team. I can't see any reason Ferarri would be interested in Niko unless they lose Vettle for some reason.
I suppose depending on what happens with the whole Renault/Lotus deal, if Renault does in fact enter as a factory works team, perhaps Niko could drive for them in their development years (like Honda atm). I could see him going to McLaren after Jenson leaves, but like the Renault idea, it seems like a long shot....although, I'm sure both Renault and Honda would love to pick his brain
So ya, all that to say that I don't see Niko switching teams any time soon. I think he has one more year to fight for a title, then he'll be a Barichello/Massa to Hamilton.