This is generally a good thing. And I am pretty darn certain that the groups in opposition to this project have nearly zero interest in horse-racing, and consist of the usual assortment of selfish NIMBY's cranks.
As for me, I have actually been to the races once. It was fun (I won $100 on a $5 bet, by total luck, I can assure you), and I'm going to make sure to get out there with my family one more time before it closes. And it looks like we're going to have to do it pretty soon, because apparently things are shutting down for good in just a couple of weeks.
What's sad is that this doesn't really have to be an either-or situation. If you look at a Google maps satellite view of the Bay Meadows site,
you see that half of the property is actually taken up by parking. Suppose we just start assuming that everyone who wants to bet on horses gets there by train--we could keep the racetrack, and still open up several acres for development. Suppose also we threw in the San Mateo County "fairgrounds", the parcel to the NW of Bay Meadows--currently just a couple of big convention-hall buildings with their own sea of parking. The convention hall function could remain, but the halls could be built with housing or businesses above, or like the Mascone Center, there could be a park on top.
When we talk about the cars, the main evils we think of are pollution and traffic, but there are other downsides to car-dependence--the perceived need for parking is a kind of tax--it at least doubles the amount of land that a business or institution requires, and squeezes some of them out of our region entirely, including ones--like horse racing--that it might be fun to keep around.