@Florenz
One would think/hope that it would be gradual.... and so, rather than racehorse being needlessly killed, or placed into possibly unsuitable homes etc etc, they would no longer be bred.
(In the same way that a lower requirement for meat would - hopefully - result in fewer animals actually being created.)
As most people on here are aware, the future for unsuccessful racehorses - and even successful ones - can be very bleak.
I was lucky enough to come across an ex-racehorse, who had had a very short career (due to his owner and jockey and some funny goings-on) - and he was the most lovely horse to have for our family, and enjoyed a long retirement with me when no longer safe or comfortable (for him) to ride. I think he had six full years just being a horse, but getting all the love and care I could give him. (A grey, so probably a couple more baths than was necessary, and certainly much focus on keeping his mermaid-hair tail sparking.)
Also though, I spent some time going to the "Sales" (not Bloodstock, although it happened there as well), and the cattle trucks/meat wagons were lurking off in the background for those who didn't reach their reserve.
In any area of the horse world, there will be cruelty, as there are cruel and evil people who want to dominate a powerful animal. I worked for top level dressage rider/trainer (and now judge I believe) - even then a middle-aged lady, who thumped and screamed and shouted at her quarter of a million pound Grand Prix prospect, just because he walked into his stable in a away she didn't like.
We - humans - can be very ugly when we seek control and domination over our fellow earthlings.