It's one thing to struggle, it is another thing to get tossed out there before you are ready to handle the job. Yes, some QBs do well even though they do struggle early on. I have seen others that don't though. From what I understand the system Rosen is trying to run is very complicated and he is struggling to get everything down. That is expected from a new QB yet it doesn't sound like anyone has tried simplifying it. Nagy did that with Trubs and he is playing well. I'm sure more things are being gradually added.
Yes, some excel but some coming out of college haven't had a chance to adapt to the professional level or the Pro System. Rosen came from a run/pass option style offense at UCLA and has to learn to play at the NFL. Some QBs are more NFL ready. And for your Manning I can throw a Brady, or a Rodgers, or a Brees. The three best quarterbacks currently in the NFL sat for their first, and sometimes second years. Some get it early, some don't but a player has to be ready. Obviously Rosen isn't ready yet but that doesn't mean he can't be.