A few years ago, an 8 year old kid sitting next to me on the plane spent the entire flight making her own animation movie from scratch on an iPad. I don't think there would have been any equivalent 10 years ago, short of drawing a flipbook by hand.
Virtually 100% of teenagers these days spend hours every week framing, color correcting, tweaking their pictures to share them on social media. When I taught film photography workshops to teenagers 10 years ago, only a very minority had any interest in photography. You can argue all you want about Instagram not being "real photography", but the reality is that it has led more teenagers to asking for a DSLR on their 16th birthday than anything else in history.
Modern touchscreen devices have always been for creating, and those who say otherwise - as Benedict points out in the opening - fundamentally misunderstand what "creating" looks like.
Now obviously, there are still gaps - no Hollywood movie has been entirely produced on an iPad yet (although quantities of Youtube videos with millions of views have, so maybe that point is moot), and you can't write an Android tablet app on an Android tablet. But that won't be true fairly soon.
I think that, in the long run, making generalizations like this hurt us. It's probably a high %, but not "virtually 100%". You run the risk of siloing discussion around just a single topic (e.g., posting pictures on social media) when in reality modern smartphone use is just as vibrant as PC use is.
People do share photos on instagram. They also use iMessage/SMS/et al. to communicate via text, Tumblr/Twitter/Facebook to consume content, and a mobile browser to find driving test locations. Mobile gaming is a thing too.
You make good points -- especially about photography and interest -- but it's not like every "teenager" falls into the same category. Their device use is just as varied as the personalities of the people using them.
> Anything that you can't do on mobile/tablet and can do on a PC is something that 90%+ of people couldn't actually do on a PC either.
However, while this is accurate, I'd argue the whole debate is irrelevant to whether or not desktop PCs will die. I mean, there's a lot of people who buy trucks or SUVs that could perfectly live with a sedan or even a smart car, but some people just prefer a bigger vehicle.
I think, too, that enthusiasts and builders alike will continue to run PCs, albeit in a smaller market, long into the future. And as it stands right now, mobile platforms are too limited to be built using themselves. That may never change because it's ideal from a security PoV, and making it possible would introduce complexities that may limit the broad appeal smart devices tend to have.
Though, I still wonder if more weird form factors, innovations input devices and/or software evolutions will change those assumptions.
To extend the instrument comparison, that's like saying consuming the game 'Rock Band' is equivalent to creating music.