Changes in Common Sports Strategies: Evolutionary or Cyclical?

Team budgets in the NFL are essentially identical. The hard salary cap and floor ensure that no team has more than a 10%-ish salary advantage in any given season.

Definitely so.

Before ~1970, the 4-3 defense was essentially ubiquitous in the NFL (and the AFL, before the merger). The 3-4 apparently originated in college football, and was adopted by several AFC teams (notably the Dolphins, with their trademark “53” defense) in the early-to-mid 1970s.

When I first started really following football in the mid-to-late 1970s, the 3-4 was used by a number of teams, but interestingly, nearly all of them were in the AFC. By the end of the 1970s, the 3-4 became more widespread, and many (if not most) NFL teams were using the 3-4 in the 1980s.

The pendulum swung, and most teams moved back to the 4-3 in the 1990s – the Wikipedia article on the 3-4 claims that, in 2001, only the Steelers were using the 3-4.

In recent years, the 3-4 has come back into vogue, and about a dozen teams are using it again.