Also--

1-- We breed many more horses than we used to, which results in many fillies being taken off the track much earlier. Less campaigns=less starts.

2-- I would argue that the horses being much stronger (whether through improvement of the breed over time or steroids and other drugs) causes them to be less sound, since it places greater stress on bones, tendons and ligaments (see Sandy Koufax, Mickey Mantle).

3-- 30 years ago most horses were raced by their breeders. Now virtually all are bought at auction, and so bred for the market-- which wants good looking, muscular fast horses. We don't breed for soundness.

4-- The stat I would like to see is starts per year per runner, and I would also like to see it for sires and trainers. Well bred horses are pulled from training much earlier, so their number of lifetime starts is less.

By the way, I talked to Joe King when I was researching "Are Racehorses Getting Faster". He couldn't tell me whether cushion depth had changed or much about composition changes at NY tracks, and nothing about other tracks. They started keeping records later, and Porcelli was responsible for them keeping really detailed records.