Ok, here we go.
1/ and legal terms horses are livestock.What that basically means that in the eyes of the law they are an economic unit and subject to such legislation. They are not regarded in the same way as companion animals such as dogs and cats.
2/ Thoroughbredd are bred for the express purpose of racing. Therefore if thoroughbred horse is not economic or for whatever reason no longer suitable for racing, they must be moved on, somewhere.
There is an aftermarket within the industry for both economally viable breeding stallions and mares. So those ones can live out their lives having sex and having babies.
There is also an aftermarket for riding horses... All the equestrian sports, stock horses, kids riding horses and whatnot.
There is however a surplus, and/or horses unsuitable for the above aftermarket purposes.
What should we do with them?
If the abattoir at was not an option there would be left to starve or suffet miserable lives and deaths uncared for in inadequate circumstances.
Therefore in those circumstances the abattoir is actually the most humane option. That is not to say that the treatment that was filmed in a recent program is acceptable, it is not in any way shape or form and there must be some serious reform there for sure.
But the emotive argument that racehorses are taking to the abattoir after racing is an agendized and overly simplistic propaganda, designed specifically for virtue signalers to express outrage.