Retro, you seem to have an axe to grind, and I'm not sure what it is.
Yes, some sports are very expensive, but many (football, athletics, rowing for eg) are not. I have a dc that has represented GB in one of these sports, and we have a very average income. The main costs are club fees (£20-odd a month), race fees (£8 or so maybe ten times a year), kit (sports direct is your friend, you don't have to spend a fortune) and transport to events (parents often share lifts or use public transport).
Once they are at international level costs go up, but they get subsidised by clubs and organisations, and they can apply for funding from sports aid and the local authority.
So it is absolutely not the case that kids from non- wealthy homes cannot compete at a high level in sport. Yes, obviously riding, polo etc are different, but they're not representative. To tar all sports with that brush risks putting people off even trying, and makes the exclusivity self-perpetuating.
And the thing that makes time off school for high level sport a different proposition to just fancying a bit of enrichment is the amount of time these kids are putting in. A top-level teenage swimmer or rower will be doing in the region of 20-24 hours training a week, spread across 10-12 sessions. That is every day after school, probably three early morning sessions requiring a 5.30am start, and half the weekend as well. On top of working for GCSEs or Alevels.
So expecting the school system to cut them a bit of slack occasionally in recognition of the amount they're putting is really not unreasonable. It's not at all the same as wanting time off for an all-round enriching experience - though in fact schools will generally give time off for those kinds of activities as well. Another of my dc has occasionally had time off for very non- professional standard choir events, and I'm grateful to the school for authorising that because it is enriching. But it's not comparable with time off for training camps or competitions for a child who is racing for GB or entering Young Musician of the Year or whatever. At that level co-operation from the school is essential to making the whole thing work at all.