It is an interesting one when you think about it.
Let's say these 8 horses raced against each other all the time.
We will say they are all tough types who race 40 times a year (makes the maths easier too)
The owners might break even if they win (& place) in turn so with 5 wins, 5 seconds, 5 thirds (5 fourths, 5 fifths, ...)
The trainer will have 40 wins, 40 seconds, 40 thirds. If he was even on 20% of prizemoney (!) that means dividing by 5 and he would get the equivalent of 8 wins, 8 seconds, 8 thirds .... He is definitley going to get this regardless of whether the horses take it in turns.
The races are worth $5300, probably only really $5k for 8 starters, if on 20% he takes $1k per race or $40k for the year under our scenario.
Consider his training fees are only covering expenses. If full time he is going to need to double this = 80 wins, 80 seconds, 80 thirds .... to make a living

Take it to the nth degree, could we one day have no private owners, just a pool of industry owned horses that go around most weeks and are trained by a small pool of industry employed trainers (and drivers) who put on the show for the punters to bet on (there wouldn't have to be any prizemoney as it would be the industry paying itself).... Yeah I know I am going crazy and it would be as boring as those - yep that is it - we will just end up betting on those cartoon races, no need for horses at all!