More don't succeed than do if they start immediately. Ryan was not that good at the start. And you named 3 guys who were drafted in the top 3 overall. Those are the cream of the crop guys.
I'm talking more guys like Bortles, coming in from a school like that. He will probably be starting by the end of the year, and that's a mistake to me. Bridgewater should even sit for a year IMO. The Texans should sign McCown to a nice 1-year deal, draft Teddy, and let him sit and learn. There is no QB in this draft who should not sit for at least one year.
I think teams not drafting QBs early when they have a guy in place is more due to the fact that most teams have a lot of holes to fill, and simply can't worry about it. It would not surprise me to see Belichick take a QB early next year in anticipation of Brady leaving. It's a very smart thing for a franchise to do.
Belichek takes a lot of QBs. the replacement could very well be Mallet.