Fishing.
"Is it really a sport when only one side knows they're playing?" (Stolen from some comedian, who I think was talking about hunting, but which applies to fishing too.)
When a sport begins to decline (bowling?), you will often see the performance drop off. If bowlers could make $10M/year, you'd see the athleticism of bowling rise really quickly once again.
Generally speaking. Disc golf has Ken Climo, who won 12 world championships, but the guys playing now (they're still not making much, but the top 10 can at least earn a living doing it professionally now) are probably better than he was, and are definitely better than Climo plus the nearest nine guys to him were on average. Disc golf, if it continues to grow, will likely see athletes continue to improve.
Though at some point we reach the edge of what human beings can realistically achieve. I don't think a human being will ever run an unassisted 2:00 mile (the record is 3:43 so I was tempted to say 3:00 mile… and by including this parenthetical, I still kind of am…).