jump racing is cruel to horses, but without it there would be more unnecessary deaths?
Jumps racing is more likely to delay the slaughter of a horse, than it is to prevent it. Those horses that don't get put down due to injuries during training or races (which accounts for a large number of them) are most likely going to end up at the slaughterhouse once they're no longer profitable as a jumps horse.
There's no retirement plan for race horses (jumps or otherwise).
Aside from their death, jumps racing also causes other problems, such as injuries from falls and pulmonary bleeding (blood in the lungs). Bleeding in the lungs occurs due to the high impact of running and jumping over hurdles. This is a problem with flat races too, but is worse with hurdles.
Jumps racing reveals the worst side of the racing industry (not that any side is very nice side). It provides a means to milk a few dollars out of horses that haven't made it or are past their peak in flats. So the reason to target jumps racing would be to end the worst of the worst and reduce the profitability of breeding horses to excess.
Hope this clears things up...