How do you upgrade when you don't know what your upgrading from. They have no benchmark yet. He needs 2-3 years especially since we gutted the roster and are starting fresh. I'm not suggesting you don't bring in competition, that would be silly, but you certainly don't look to replace him with a first round QB one year into it.
They will have a pretty good idea on Rosen after this season. They will know if they need to try and find an upgrade in next year's draft. They won't be willing to wait for 3 years to see if he has it or not.