Waiting for some responses to my theories on tennis.
It carries some implications despite this proviso: "These changes formulate different tennis answers for success—this is not to say that each subsequent overarching discovery on tour and its slow filtering down to the field leads to a base answer that is superior to all previous ones—though, as a general rule, sports do evolve."
Rarely does the tennis field really get "weaker". It can occasionally happen in sports where a bunch of legends retire and there is truly a giant drop-off in the strength of a field. A good example of this is in F1, which has a small roster anyway so if a few legends happen to end their career then it's going to have a huge impact on the field.
From about 1993-1994ish (can't remember the exact years) F1 lost Prost (4x Champ), Senna (3x Champ) and Mansell (1x Champ). Suddenly the field was severely weakened—the dominant palaces had collapsed leaving merely rubble in their wake; and a new land to be terraformed by those who dared.
It's hard for this to really happen in tennis. When Sampras retired in 2002 he was far removed from his prime and we still had Agassi living it large yo. The new pretenders had already become contenders with the likes of Hewitt, Safin and then Roddick blooming rather early.
The strength from year-to-year in tennis can meander, but along what is generally an upward slope. I imagine it as plotting a path of changes in tennis and their responses (typically from the best players) against a path of realisation from the field on a backdrop of evolution in tennis that always slowly trends upwards, disturbed by localised noise from the arrival of new players and the retirement of older ones.