In our last thread on the topic, @meili_yinhua claimed that the 9 stone rule was implemented to prevent extremely slow rank settling:
And indeed when I simulate someone who only plays well above both their “system” and “true” ranks the descent towards the true rank (in this case it’s set to 25k) is significantly slower:
In order to prevent this type of thing, it might make sense to keep a rank-difference restriction, but make the range a function of deviation (in other words, new players can play anyone, less new players more limited)