Paraphrasing: "If you don't want to, don't sideload apps, no one is forcing you to" - do people really not see the problem? It's not that technically unsophisticated users will want to sideload apps. They don't know / don't care / have different things to worry about.

But they want their Facebook. Or SnapChat, or Insta, or TikTok, or whatever.

Once other app stores are allowed, there's nothing stopping Meta (for example) from revoking their existing apps, and requiring the use of the Facebook App Installer for access to Facebook. They've paid people in the past to use their Onavo VPN app to bypass Apple's privacy controls, so this would be unsurprising.

Of course, that's just an example; replace with the next SnapChat, TikTok, whatever. If that's the thing that teens want, and the way to get it is to click a bunch of "Yes I agree" dialogs, they'll happily do it. And now suddenly some developer has access to all your family financials through your teen.

If your solution is "well, people shouldn't do that then", you might not understand teens. (Or grandparents. Or regular people.)