Did we read the same article? It spends so many words answering these exact questions with examples and helpful illustrations!
Your question:
> why should they care that they don't "own" their Instagram post, whatever that means?
From the article:
> The web Alice created—who she follows, what she likes, what she has posted—is trapped in a box that’s owned by somebody else. To leave it is to leave it behind.
On an individual level, it might not be a huge deal. However, collectively, the net effect is that social platforms—at first, gradually, and then suddenly—turn their backs on their users. If you can’t leave without losing something important, the platform has no incentives to respect you as a user.
Your question:
> can you give examples of good and bad incentive structures in this context?
From the article:
> Maybe the app gets squeezed by investors, and every third post is an ad. Maybe it gets bought by a congolomerate that wanted to get rid of competition, and is now on life support. Maybe it runs out of funding, and your content goes down in two days. Maybe the founders get acquihired—an exciting new chapter. Maybe the app was bought by some guy, and now you’re slowly getting cooked by the algorithm.
> Luckily, web’s decentralized design avoids this. Because it’s easy to walk away, hosting providers are forced to compete, and hosting is now a commodity.
I think you’re right that the average person doesn’t care so much as they just want to be entertained or reach a large network, but apathy is not an argument in favor of the status quo.