I completely agree with your last paragraph. And I think that voluntary participation would at least be a huge improvement on any compulsory ritual athletic events with dozens or hundreds of people watching.
The trouble is that even then, it singles out a particular group of kids for celebration that others don't get, and it takes up half a day or a day of school time (and an awful lot of organisational time and effort) for something with few tangible positive outcomes that couldn't be achieved in a lot of other ways. And while a lot of kids might enjoy aspects of the "sit and cheer on" role, it's basically a doss day chatting to your mates, if you're lucky.
Posters on these threads often say it's some kids' only "time to shine" and that that alone justifies the whole hoopla. But when does the star food tech or phonics or maths kid get to "shine" in front of all their friends, all the other kids, all the teachers, and maybe their parents and their friends' parents? Seems a bit of a thin consolation to say "Yes, Ricky always wins all the races and gets his "time to shine" where everyone has to watch him win stuff, but darling, he's a bit thick — while you'll probably be getting marvellous GCSEs in several years' time, and you'll get to open an envelope with your grades in! So, you see, it's only fair."
Cause that's what they mean, posters who talk about "time to shine". They seem to have this bizarre idea that academic ability and sporting ability are negatively correlated (it's actually the opposite) — and the second bizarre idea that everyone must do sports day so that those poor unfortunate bottom set kids can win some races, as compensation for having to study academic subjects they struggle with and being doomed to poor grades and thus lifelong failure or some shit.
Anyway, returning from that slightly elaborate tangent about the ugly and weird assumptions behind "time to shine"…
Yeah, if you're happy with multiple compulsory-attendance subject-stars celebration days every year, each focusing on a small number of children with a particular enthusiasm or skill at a particular subject while the others look on and make the appropriate responses, then I suppose at least that's fairer (though still tough on the kids who don't excel at any one particular thing). But I don't know how you'd justify the necessary admin and lost learning time, nor how you'd design a full-length public appreciation event for the kids who absolutely slay at close literary analysis, or computer science, or biology, or, I dunno, Latin.