I think the problem you run into is timing - you get a four-year window where your QB is cheap, plus a fifth below-market year, maybe a sixth if you backlog an extension. So if you draft a young QB with the intention that he sits for a year and then starts, that works. But if you draft him into an uncertain situation where your starting QB might be there two or three seasons, it really eats into that cheap QB window. Brady, Brees, Rivers, and Roethlisberger are all signed through 2019. Is one of those teams going to use a first-round pick on a QB to have him potentially sit for two seasons? Maybe more (given all four of those guys are still good-to-excellent)? It probably makes more sense to look to the 2019 draft (which as I understand it is not considered QB-strong, but none of these four teams is likely to have a crack at any of the top-five QBs this year anyway).
What will be interesting is seeing if teams like Miami and Cincinnati who have decent QBs on expensive contracts start punting on them and going younger, like KC did this year drafting Mahomes. Does this then create a relative glut of competent veterans, depressing salaries in that market, or will there always be more seats than players to fill them in this game of musical chairs?
EDIT: have you read Zach Moore's book Caponomics? He talks a lot about this kind of stuff.
