And maybe he can play well as a pro, but it's all up in the air, and he hasn't proven anything other than he was a good college QB. Dak has proven that much - again, better stats and team record - and has proven more as a pro.
And, again, I understand it wasn't a good situation in AZ last year, but that doesn't mean he is very good and only the team was bad, it only means he is still unproven.
In the end, he is still a prospect who hasn't shown to be as good as Dak in either college or the pros, much less better, much less significantly better.
As for your question of why he has to be better for us to be interested, I've indicated that clearly and repeatedly. Yesterday I wrote the following, and you responded to it so I assume you read it.
If your point is he would be a great backup, and it would be great to see if he can develop into something more than we know now, I wouldn't disagree, but that's not a stand alone point. You also have to consider cost. Do we really want to give up something significant in terms of draft picks for a player that may well end up as nothing but a backup? I love the idea of a young, developmental QB and a strong backup, but a team that already has a young starter who is winning and productive while he matures looks for that in middle rounds, not at the top of the draft, nor do they trade away a top draft pick for it. That's my biggest issue with it. If we could trade away a 3rd or 4th round pick for Rosen I would be all for it, or maybe even a 3rd and a 4th spread over 2 drafts, but I think the asking price for a guy one year removed from being the #10 overall pick would be much higher than that.