We started with Kumon maths because DS wasn't being challenged at primary school and we hanged in there because we were aiming to get DS into a selective secondary.
If a selective secondary wasn't our objective then we would have stopped at the end of Year 5. Otherwise, DS would have gone into Year 7 and be bored because he be doing stuff he did in Year 6 at Kumon. Better to be bored in Year 6 than in Year 7 was my thinking.
As for reverting, the study ethic will have been instilled in your DC. That isn't going to revert. DS did find that his mental maths wasn't as good as before but it still better than mine.
Either way, we wouldn't have carried Kumon maths onto secondary school since there are better resources out there for secondary school maths.