Even if that ends up being the decision, I doubt that it has been made yet.
The only way I can see them putting all their eggs in one basket like that, is if Rosen gets some playing time late in the year, and does well, not only making the occasional "highlight" throw, but operating within the framework of the design of the offense. He was obviously not able to do that earlier this year.
To be fair, the line was horrible then, and him being indecisive, and a second slow making decisions, put him in a situation where the opposing pass rush was eating him alive.