Well Rosberg won a race this year. Is there any race Michael could have won this year even without DNFs. I just looked up some info online and it seems Schumacher is not retiring at the end of this year, atleast not volunteeringly. That case the only way I could see Lewis joining Mercedes (if he even wants to which I am far from convinced) is if they dump Schumacher on their own accord in favor of Hamilton. I dont think Rosberg is going anywhere anytime soon.
Red Bull isnt as dominant as the last 2 years but I havent heard anyone suggest it is slower than Ferrari this year. Alonso in fact has said he thinks his car is only the 4th or 5th fastest this year, not only slower than Red Bull and McLaren as most feel, but slower than Lotus and/or Mercedes as well (though that could be trying to make himself look good).