There are approximately 43,000 Greyhound races in the UK each year and (as a round figure) 245 deaths directly occurring during or directly after a Greyhound race and approximately 4500 injured during racing (mostly treatable).
So a very very small percentage of deaths directly related to racing.
You are correct many pups don't make the grade often because they are "fighters" and not just too slow,in fact since the regulation of the FOBTs in betting shops (limited stakes to £2) the number of Greyhound races has increased hugely (most tracks are bookmaker owned) and trust me plenty of Greyhounds that wouldn't have "graded" years ago are racing every morning or afternoon nowadays.... and they are racing way to often (average of once every 7 days) the clue is in the number of sprints in bags meetings now, never used to see them.The trainers who are contracted to the Bookmaker owned tracks are now not reliant on gambling to make a decent living anymore, average £500 a day income for Bags meetings.
Personally I dropped out of the sport years ago because I refused to allow my dogs to be run at the behest of Ladbrokes for their "bags" meetings at Hall Green.... they want it all their own way ,I buy the Dog pay the training fees etc and those cunts wouldn't lay me a bet on my own Dog.in fact the person that trained my Greyhounds quit because he felt he had become a van driver for Ladbrokes plus he wasn't able to bet on any Dog he trained, absolutely ridiculous,(he was my friends Son and won the British open race trainers championship more than once),his Kennels are now just boarding kennels.
Is the sport cruel?, well yes of course some aspects are , there is money involved.
Very long winded reply and I could tell you plenty more tbh.
But lay the blame at the corporate bookmaker industry... simple.
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Edit: Just for interest tbh years ago I used to take absolutely top class Greyhounds to run at Hinckley "flapping" track for a trainer and on one occasion came up against a Greyhound Derby winner !!! , strong betting market at Hinckley.....I loved it.