I do not think it is really difficult at all. I was stretching the definition of franchise QB to make that work but even stretching it calling 2015 Manning a franchise QB is a bridge to far when Geno Smith is unquestionably a better QB now than Manning was in 2015 and Geno is not a franchise guy. You can find a Eli Manning in the draft fairly consistently in round 1 and I would say you could find someone to do Russel Wilson's job in round 1 easily enough (hand the ball to Marshawn and let the defense win).
So lets reframe the question just to showcase this, of all the teams to make and win the Super Bowl this century, how many of said teams were driven by the QB and how many was the QB a glorified game manager? Once you deal with the effect Brady has on the stats, you see that it is not common for the QB to be elite at the time of the Super Bowl or even the driving force of the team. Basically for most of these teams you just needed a QB playing at the level of an average first round pick and if that is case it might be better to chain together rookie deal QBs, be loaded on first round picks and have the money to sign top tier free agents to supplement said roster.
Yeah so I have many issues with this thought process:
1. The entire league, in total, could find a franchise QB every 4-5 years consistently. But in your scenario, you're extrapolating that to assume that a single franchise can, theoretically, churn out a mid-tier franchise QB every 4-5 years consistently.
The obvious issue is that, historically, we know a franchise can't do that. Because we have a lot of franchises that you could make a case haven't had even a mid-size franchise QB in 20, 25, 30 years. Certainly not from lack of effort, because those same franchises have drafted many, many, many QBs in that time, signed others, and it didn't seem to matter. High picks, low picks, traded picks, etc.
So in order for that scenario to work, you'd need to show me a franchise that can consistently compete, in 4-5 year cycles, while changing QBs in that window.
2. The other glaring problem with this discussion is that you're assigning "Pass" as a SB trophy, and "Fail" as literally every other outcome. I don't think anybody can stand behind that opinion logically. SBs are hard to get to, let alone win them. So are Conference Championship games. All perfectly fine metrics for success.
So a more prudent model would be to assess the overall success of that team during the example of a 4-5 year window.
3. The main reason why NFL franchises aren't doing this, and likely won't, is because NFL teams are run by people who like to collect paychecks and keep their jobs. So the first time that a franchise opts not to keep a high level franchise QB, and instead decides to invest on a mid-tier player (via draft or wherever), and that doesn't work (which it frequently won't work), the person making those decisions, along with many other people, will get fired.
But at a high level, the biggest fallacy with the thought process is that you're assuming that all NFL franchises are actually competent enough to obtain mid-tier franchise QBs on a recurring basis in a 4-5 year window. That's not an opinion that has any real basis.