I think the fundamental problem is that from very early on in human development, survival even, people have made use of various animals in an effort to make their own survival a tiny bit easier. Other animals do this too, as do plants and fungi —think symbiotic relationships. Over thousands and thousands of years, human uses of other animals have refined and extended and that's why, among other things, we have stuff like horse-racing. It has various attractions and excitements from betting to the whole industry of breeding and training.
Which is why we can't just shut it down quickly, even if it is cruel. I presume the horses that are euthanised are eaten — dog meat — though I wouldn't be averse to eating compassionately grown horse meat. People do in other countries.
If the RSPCA has not seen fit to revise its article since 2006, perhaps that's because they didn't think it needed revising. That, if it is the case, would be encouraging, meaning that things are not getting worse and increased care of race horses is being effective. On the other hand, RSPCA could be in the pay of race horse breeders. Take your pick.