I was reasonably sporty in all but running, or more accurately, running fast. In a 100m race, I would always be last, despite giving it my all. I could hold my own in cross country, or even 800m, probably because I was fitter than many of the others, and that plus my determination eoukd see me through a longer distance.
Luckily, in secondary school anyway, we could choose what sports we competed in. That led to what I still fondly think of as my finest hour - winning the high jump one year, and not only that, but beating my nemesis, a girl who had been nasty to me since the first time we met in primary school as six year olds.
To force a child to participate in a competitive race against their will, on sports day, in front of an audience, knowing that they will always do so badly that they will be humiliated, is cruel. It's bad enough during the normal PE classes, but there at least I agree with the argument that 'we all have to do things we're not good at / don't like', and is quite different from a compulsory competition.