The thing is, the royal ballet school is one school, a very prestigious school, yes, but only one school. My dd has a friend in y7 there, and she was a JA. She also had exam grades that showed her potential.
In my opinion, if you want to be a BALLERINA, then yes, JAs would probably suit you, and your ultimate goal would be a place at the RBS. However, in a way that is closing more doors than it opens, and limiting you to one genre (ballet). A few years down the line, when you suffer an injury, or you grow too tall, your career is over before its begun.
Some of the other vocational schools and colleges are therefore much better at keeping your options open and who truly knows, at 11, what they want? I have seen many JAs have their hopes and dreams crushed. I have seen many children who would not have been accepted by the scheme go on to be amazing performers. I know several in the west end as I type, a couple in ballet companies around the world and many excellent teachers too.
Also, for the record, at 16 most vocational schools will be looking at the exam grades achieved and including them in their decision making process. Usually the latest exam results, but they do matter. And they get UCAS points for them too.