I had no idea that the idea of nine-dan was used outside of go across all kinds of disciplines. Do you have some examples? (I hope I don’t sound cynical, I’m genuinely curious.)
I guess they CAN be defined, but why should they, especially when it comes to go? The consensus seems to be that there is a lot of meaningful distinction to be found beyond the site’s current rank caps. Why not keep with the current ELO-to-rank conversion and let it naturally determine what the lowest or strongest ranks of players on this site are, instead of pre-defining them as 25k and 9d and then retrofitting the ELO into those boundaries? As has been pointed out, this would change the current dynamic of “one stone per rank”, for the sole purpose of keeping these (arbitrary or not) rank caps.
But wouldn’t it be even easier to keep the current method of ELO-to-rank conversion, which provides the same continuous sense of progression, and remove the artificial caps?
If I understand correctly, your argument is that human abilities have a lower and upper bound, and that the ranking system should reflect that. You feel that 9d should remain the upper bound for traditional reasons, but you agree that the lower cap of 25k is arbitrary.
I’d argue that the reason that the ELO system shouldn’t be fitted to the ranking system is that the best players (which include bots like Leela) are continually getting stronger in ELO, meaning that everybody’s ranks would have to be continually adjusted to make sure that only the strongest players have a 9d rank attached to their ELO. The ELO rating of these players would continue to rise but their rank would stay 9d.
Not only that, but the ranks of players with stable ELO would constantly go down in the face of 9d players continually getting stronger. I think that it would feel much more natural to have strong players’ ranks go up as their ELO rises. The current system of translating ELO into a rank makes much more sense than translating a rank into ELO, in my opinion.