It's an interesting debate - whether high level music achievement carries forward to adulthood. I am amazed at how many adults (young and my age group) that I meet, who played instruments to very good levels, but have never touched them since leaving education.
I think that not only does it depends on the child, but also how entrenched music making is amongst other things they do. If your friends whom you spend every day with, are also those who you make music with (formally and off-line, so to speak), then then there is, perhaps, more stickability to this carrying on into later life. A bit like eating a food and it instantly taking you back to a fond memory. Might make you want to cook more, because you reminded constantly about the pleasure you got. Associated links that our DC create on this thread, through their music groups, NCO/NYO/camps, ensembles etc etc, all provide positive connectors and entrench more if you are doing it with 'your type of people'.
I'm sorry, I'm babbling, but see this in my DD's swimming world too. Those that get that the sport is both an individual and a team game get the most out of it (as well as being around people like them, who can't party all hours, cram homework, do same training hours etc), often get each other more and are also the most likely to take up a masters class etc, or swim early morning up and down a lane because they've learned how de-stressing this is for them, just like tinkling or improvising on piano keys for hours when they should be practising their scales can be, or jamming with their friends. This on top of their 'day' hours. The camaraderie with any specialist learning can help alleviate periods of highs and lows in motivation, helping to nurture a love of learning something hard (piano, swimming etc) vs just going through the motions, albeit to high standards, like rote exam taking leaving some cold.
i may be wrong Druid, but having attaching strong values to piano learning from the start, as well as your DS finding his people early that are, bonus wise, also his music people helped cement his love because they are both entwined. Not to say that his passion and determination would not have been achieved alone, just like Mistri's, albeit in later teenage years. Just both DS' seemed to engage really well with their music with both a hands on and hands off approach and early journeys, resulting in similar drives.
All that, to say I don't know what the answer is. I know I used to say there was more we could do with our money if DD didn't want to progress with something, but not to disrespect the teacher of said thing either by not practising. Making it her call, pointing out the possible consequences was all I could do. I was never tested on it Green, and by the sounds of it, your DD has not said either that she wants to quit. Once one stops comparing where a child should be (vs MN, RL kids etc that are excelling), and just let them learn at own speed, a natural love or hate will fall out. Few years back, inside I would be dying, thinking how many hours does this girl have to spend in the pool before she realises she'll never catch up with the others and is not that great. And now, I'm a little embarrassed to not have seen not only the late developer, but the extra value she got from it all, but needed my patience, for her to be patient.