Bear in mind that I am no mathematician and have only a temperamental old calculator. Nevertheless, I tried to work out a few figures.
According to Wikipedia there are, on average, 94,000 horse-runs in Britain per year. (No of races x no of horses in them.) According to the OP, some 170 of these runs on average every year (1000 divided by 6) result in a horse's death. That seems to give a death-rate of 0.18%
According to Government statistics, in 2010 there were 493,000 human deaths in England and Wales from a population of some 57,000,000. This gives a death-rate of 0.86%.
In any given year, racehorses seem to have a much better survival-rate than WE do!
Even if my maths are all to pot, I simply must reiterate, if there was no racing there would be no racehorses.