I don't understand. How have you determined the standard, or the level of pleasure derived?
From long personal experience of playing musical instruments. If you like art it's a lot more satisfying to be able to draw a man that looks like a man than a dog. It's the same with instruments.
If you're tone deaf you might not notice if you're shit, but most people do. Most people have a general grasp of what standard they are. Playing out of tune, not being able to get your fingers round the notes, not being able to play repertoire you would like to, is not particularly enjoyable.
You do get deluded talent show contestants who really enjoy belting out a song sounding like a dying cat. How much does everyone else enjoy it? Do you really think objective standards cannot be determined?
And what of those children than don't take the 'traditional' route, but can knock out jazz, or other non mainstream genres with far more pizazz than a classically trained, let's go through the motions route?
Why do you assume classical training is 'go through the motions'? In fact that is the way of bad teachers whatever the genre - jazz, classical.
There are some children who are exceptionally gifted who can pick up an instrument quickly whichever the genre. But it's entirely dependent on their own talent. There are plenty of children who are enthusiastic but not so gifted or confident, who with good teaching and support, could reach a good standard.
We do do it your way Twinklestein for the record, but cannot say she takes enjoyment from all of the chamber music she's subjected to, especially when most of it is really dull and catering for too many levels of ability
?? I'm not sure what 'catering for too many levels of ability' refers to. If you think chamber music written 'dull' I'm not sure why you're bothering. A lot of chamber music is very beautiful.