The National Football League is touted as having parity. Any Given Sunday. Free agency, salary caps, and revenue-sharing are some of the means whereby the better teams are reined in so that anything can happen on any given Sunday. Even teams with poor records are rarely blown-out consistently. Advantages from innovation are short-lived due to copy-catting. You would expect an NFL team's success to resemble other generally random functions, replete with mini streaks and constant reversals of fortune.
Deviations from random should correlate with protected attributes such as rookie mandatory salaries. Since 2002, a team should win the Super Bowl every 32 years, its conference every 16 years, its division every 4 years. But, other teams may concentrate their resources in a certain year so that your team is not up against a similarly random-quality team. So a season might not be the best interval since a general manager can optimize over several years with salary cap considerations. In this case, an NFL general manager would be wise to pick a certain year or maybe two year range, then time the team's resources to hit on all cylinders. This strategy is at the cost of having some down years since the team would be rearranging the timing of its strengths.
Bottom line: a general manager's main objective is to identify the best season to peak, based on what other teams are likely to do; balancing talent over each year is a recipe for mediocrity when it comes to winning titles.