There will always be horse racing as long as there are horses and owners - it’s just human nature.
Horse racing isn’t “declining” so much as it is consolidating, and in part, this is because of how much land a race track takes up (not just the track itself, but the grandstands and stable areas). Reducing the number of active tracks not only lets the owners sell to developers willing to pay premium prices for the land (case in point: Bay Meadows), but reduces the amount of “down time” the remaining tracks have. Even at that, I know two tracks - Santa Rosa and Pleasanton - that are dark 48 or 49 weeks out of the year.