I don't know if we can ever get a Rivers/Warner level talent, but if we do great. I am more concerned with his HC career than his OC career. People complain about CPJ because he apparently only focuses on his offense and it is wins that matter. So sorry if I am skeptical of a guy who has not proven to be a winner as a HC despite proving he can put together an offense given a HOF QB.
I don't agree with your first point. Obviously just comparing talent levels the likelihood that we get a successful NFL quality QB isn't slim, as it is for almost any school. It's just the numbers game. But comparatively speaking Roethlisberger, Rivers, and Warner were all very good, but none were ever really the tip of the top QBs (IMO guys like Brady, Manning, Breese, and Farve). Maybe I'm wrong since I'm not the biggest nfl fan, but I think that's the case. And at the college level I think it's unlikely that we can attract that absolute top of of the bunch (maybe in the future if we had several success stories), but I do think we could land a tier 2 (read still very good) type of guy.
My issue with the HC argument is that, as people have pointed out, the NFL and college games are different. That isn't just true for saying that his accomplishments should be viewed with a grain of salt but also that his shortcomings should too. There are many ways to fail at being an NFL HC that aren't as big of an issue with being a college HC. It also isn't like his HC career wasn't without successes. His time with the Titans sucked, but he did a pretty good job with the Cardnials initially improving them right away. Eventually it didn't work out, partially because they never could get a solid QB after Warner retired (one difference where the NFL and college is different),
To me the biggest sell for him is his work with QBs and his biggest downside is being away from the college game so long. I actually don't view the HC record as being overly significant because of the differences in college and pro. I like the idea of him being the one to help develop Graham, and think, like with pretty much any hire, that assistants will play a huge role in how successful he is. He isn't my first choice but man we could do a lot worse.