I would sound a slight note of caution. Even the most talented, committed musicians (I was one) can reconsider. I pursued music doggedly till leaving school, then realised a career in it wasn't for me. My sister was an amazing precocious cellist and was offered a place at the Menuhin school very young. My parents turned it down and thank God, because she grew up to become a lawyer who likes music but certainly doesn't want to earn a living from it. We were both glad to have not just academic back-ups, but other interests, hobbies and a broad education.
There'a a lot of truth in that, but the problem is that the obsessive focus on music in youth and having the "choice" to do it as a career or not, are two sides of the same coin.
The fact that music is so incredibly competitive, poorly paid and hard to make work as a career makes one want to leave "options open" as you say. If a child develops at some stage an equally passionate interest in law or engineering, and are clever, it's hard to ignore the fact that they'll be much more likely to have a securing and fulfilling career doing one of those. And if a child likes other "arty" things like drama, it's hard to avoid the feeling that being able to enjoy taking part in all those is probably better for their personal development than practising four hours a day.
But OTOH, it's precisely because music is so incredibly competitive, poorly paid and hard to make word as a career that it's important to do the obessive thing in early childhood if a child or their parents think they might want to. There are two possible answers to extreme comptetitiveness - to walk away and decide it's not for you, or to enter the competition, bring every ounce of force you can to it and win. Either is valid, but anything in between isn't really.
You say for example that your sister's place at the Menuhin school was turned down and then she became a lawyer, but who knows? Maybe if she HAD gone there, and been surrounded by incredibly talented and dedicated musicians through her childhood, she would have wanted to become a musician, and become a happy and successful one? I'm not saying she would, only that these things work both ways. We make choices for our children to give them options, but those choices also determine, limit or bias those options.
With most professions, you only really need to start thinking about what a child will do when they come to choose their GCSEs. If by that point they're showing passionate interest and ability in medicine, art or running a business, then you go with it - no need to worry too much about it before that. But with fields requiring high levels of extremely detailed physical training like music, ballet or sport (and all of these being highly competitive fields with not much living to offer anyone below the very top), it just doesn't work like that.
Noone ever took up music at 16 and became a professional. OTOH noone knows what they'll want to do as a profession when they're five. What do you do? It's a judgment call, I suppose. My personal view is that schools like Chets and Purcell are academically strong enough that they won't really stop someone from doing well in another field if that's how they end up. Their results seem to be well above average for comprehensives and closer to those of some grammars or private schools. And it's not like somebody can't take a year out and go enrol at a college to get another A Level if they need to. I think any possible "damage" to a kid's prospects in a non-musical field from going to a music school is likely to be less than the potential damage to their prospects in music from turning down the opportunity at such a formative stage.
Also with the new tuition fees regime, I think people probably really need to work out whether they want to do something as barmy as music (and by "want" I mean, "can't conceive of not doing it") by the time they're 18, not 22 after a degree and 50 grand's worth of debt. If these schools have high drop-out rates, then it's probably no bad thing that kids are getting the chance to find out for themselves what being a musician is really like good and early, while they still have time to discover it's not for them and move on.
IMHO.