"Then you have parents who decide that they know better and teach (I mean actively teach, either themselves or with tutors or whatever) their DC the school curriculum from years ahead to their DC, thus causing problems with boredom in school, then demanding that the school continue the ill-advised acceleration or teach an entirely different curriculum to their child to the one that is happening in the classroom, the one that it is the teacher's job to teach. This is something that you don't see with other subjects."
To be fair, noble - and I agree with many of your other points - if you are a parent with relatively limited maths (apart from school maths) and have an unusually able child in maths, then the school curriculum is the only maths you know, and the only provision for their more able children that they can imagine.
If you are a teacher, or were a more able mathematician, or did Maths beyond school age, then it is significantly easier to identify possible avenues that your DC can explore, and ways of accelerating them that are not linear.
In the same way that my school has pointed a very able singer towards avenues to explore to maximise their potential, maintains a list of extra-curricular provision for a variety of sports to a high level etc, then I do see it as the school's job, as well as saying 'don't just look at linear acceleration', to actively signpost able children and their parents towards ways to extend Maths outside the curriculum. You can't just say 'don't go that way' without giving any indication of the preferred route IYSWIM?