I think you should replace "capability " with "talent" or lack of.
There are lots of things you can become good at, even shy/unconfidant people can learn to become good speakers in front of an audience, or someone can learn how to run a successful business,especially as they get older. But some things just come down to pure talent, particularly around performing arts such as singing and drama, and even the best of those don't always succeed in it.
That's when you need to be honest with your dc, I don't mean crush dreams! But be realistic. I have advised her not to throw away the two years she gets when she leaves school which which will be the last free education she gets, and to use them as a stepping stone. I've also said to her, "Try to learn skills which are transferable."
My dd enjoys drama and does it at GCSE. She's fairly middle of the road in it, doesn't sing or dance or do clubs out of school, etc, but enjoys it. When it comes to doing her options for college, we discussed what she might like to do. Some of her friends are doing drama alongside some other random subjects (I say random because they don't really "fit" together as anything particularly useful). I have discouraged her from doing drama at college because it's difficult to get a job, you have to be very resilient at handling rejection and criticism, which my dd isn't great at, and most who do well have generally been doing drama/dance clubs for years and have a certain drive. Plus we don't live in London or a big city. If she was desperate to do it, I would support her, but I also want her to be realistic.
I come from the generation where every Tom, dick and Harry went to Uni in the 00s and did silly courses, which is why degrees became common instead of a thing to really be proud of, and why they had to start charging high Uni fees, because it was getting ridiculous as people weren't using their degrees. This has made it harder for my dds generation, so I want her to really choose carefully whilst her education is free for the next 2 years rather than just pissing about. Her generation can't, unfortunately.
She has chosen what she wants to do and is happy with her choices.