Lots of good points here; I'm thinking that much of our disagreement with Rosen is semantic. Most QB coming out of college are developmental. There are very few college QB who come out and light it up as rookies. I think the upside on Rosen is considerably higher than that of Grier, Stick, etc...and I still think the chance of his success is higher than you're implying. I certainly like his future in the league better than Josh Allen's.
However, I think the 2020 QB options are considerably better than they've been in years - working under the assumption that Tua, Herbert, Fromm and Love all come out. Even if Rosen plays well and shows signs of growth, unless he shows beyond a shadow of a doubt that he's the QB of the future for Miami, they need to draft a first round QB.
What I really don't understand is the narrative that trading for Rosen was a waste. It simply wasn't. I'll go even further than that...Josh Rosen never should have been available to Miami. Steve Keim should have heard the report that when Rosen said, "If you draft Kyler Murray, I'll just beat him in camp," and said "GOOD." He should have let them compete, the better QB starts, the worse one develops. Now, if Murray bombs, and Rosen flourishes, Steve Keim is going to be out of a job next year.
There is absolutely no reason, (again, short of Rosen completely tearing it up) to not draft a QB in round 1 next year, let Rosen compete for the job, and the loser is the backup. At worst, you've got a serviceable QB and a more than serviceable backup for a bargain price. Hopefully, you've got your Franchise QB and a serviceable backup. Best case? You have two QB who have shown franchise potential, and one of them is a very desirable trade target for another team.
I agree with everything you said except the bolded part , . . . depending on your definition of "tearing it up." If he shows top 10 talent and Flo sees a higher ceiling, that IS a good reason to skip a QB in '20. Teams with good QBs don't go drafting a QB every year. I still think, if Rosen shows he's a solid QB and shows improvement throughout '19, there'll be no '20 QB. OTOH, if he shows he's Henne 2.0, . . .