Interesting thread. And so subjective. It is very much child dependent on whether activities are too much or manageable.
Music is subjective too. Not sure I understand why Bert's son is not grade 6 equivalent, just because he's not sat exams. Many top conservatoires are full of people who never sat a grade in their life. And not everyone aspires to take music to higher levels, no matter how good they are but enjoy it at the standards relative to them.
I do personally treat music like other subjects though. If my daughter has chosen to do piano and violin, and I fund these, and her teacher has asked her to practice x, y and z for next lesson, then I expect her to do this. She'd never dream of turning up to a maths lesson not having done her homework, so why would her music teacher not command the same respect. Or alternatively, give them up if this deal cannot be met.
I agree that keeping a child motivated is key, but there is always an element of grind too along the process (exceptions may be those just naturally gifted and intuitively 'get' their instrument). So, whilst it's nice to knock out flashy pieces, the dull stuff like scales, studies etc are necessary, if she wants to be a better player long term. Same can be applied to most activities.
We're a swimming household also unfortunately. In the OP's case, if he's already indicated he'd preferred the fun aspects vs the gruel of squad training, then I would go with this, as the increased hours are arduous, full of highs and lows and really dull for a kid that's not into the training bit.
But, if your child does get the bug, then the stamina, resilience and perseverance building , team bonding etc does have it's benefits, if you are prepared to take the suffering as a family!
They do change a lot, so retaining a balance of throwing a towel in on an activity prematurely (because of the many brick walls) and pressures of other stuff like school homework, down time, social life, clashes etc is a fine line, and one personal to your own family circumstances.