Nobody sideloads, especially not kids

Kids are technologically clever, and so long as one of them figures it out it's gonna virally spread to the rest of them. That's certainly how it was for me in school.

So, I suspect that lots of people are going to be sideloading in the very near future.

If my young cousins in a 3rd world country are any indicator, sideloading will lose its "fear factor" because these kids are doing insane things with devices these days, stuff I wouldn't have been able to nor had any will to do in their day. Even more so the ones in 1st world countries!

e.g. I agree with you.

Definitely less "insane things" these days than back in the day. I remember jailbreaking Sony PSPs for all my classmates in middle school :D Later when iOS became big, the only common "insane" thing was app store account sharing.

They're also fickle, so if you lose momentum for long, your target audience may move on.

I don't think that is quite right. Fdroid and the Amazon app store are two prominent examples. While they don't have the same impact as the first party store, they also aren't Fortnite.