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).I've got something coming out in the next day or so over at Pro Football Weekly on this, and why we might see six first round QBs this year. The economics are such that teams can take the risk on a rookie QB because if they play well, they're cost-controlled and you can be aggressive addressing other needs, and if they play poorly, they're easier to move on from than say a huge free agency bust. Last year, half of the playoff teams had a starting QB taking up a lower percentage of their team's salary cap than the league average (league average being 8% of a team's cap). Four of those teams (Jacksonville, Tennessee, Philly and the Rams) while the other two had a free agency bargain (Buffalo and Tyrod, Minnesota and Keenum).
The Seattle example is a good one. You can look to the Rams this offseason as well. Goff accounts for just 4.15% of their cap next year, and look at the players they added this offseason. At some point that bill comes due, but until then you can really maximize that window with a cost-controlled QB.
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.