And who is the arbiter of "within reason"?

For me it would just be “complies with the laws of the country”

An acceptable compromise I think would be to have unlisted apps that can’t be found on the top charts or in search but can be installed via the App Store with a link.

You can already side load apps on iOS, you just need to go through some hoops (install Xcode, compile the app, create a developer account, futz about with certificates) but you don’t have to pay any money (you don’t have to pay the $99/year, that’s only necessary if you want to sell apps).

Well, at least you have to purchase another device from them to run xcode on. Those 99$/a aren't the biggest hurdle of you aren't a Mac user. (even assuming a skillset that includes "futzing about with certificates")

Applications installed like this have limited functionality--such as no push notifications--and some types of application are simply not supported (such as VPN clients: Apple seems to have structured everything in order to best support the China Communist Party's control regime here). You can only have three such self-signed applications installed at once, and you have to keep reinstalling them every 7 days (which is more annoying than you would expect if you haven't really had to do it constantly).

And if installation of apks from unknown sources goes away because some nasty malware exploits it or other security excuse becomes available?

Hard not to be alarmed by the pattern here.

Samsung does (or at least did) lock down their Android phones like that. They had to be rooted to get third-party APKs installed.

Do you have a source? I can’t find any myself. I (and others in my family) have owned several Samsung phones and have always been able to install APKs, so this would be news to me.

Did, then. My first smartphone was a Galaxy S in 2010, which was locked down like that.

Edit: Maybe it was AT&T, not Samsung, but if so that's a broader scope anyway - and my current AT&T Android phone isn't locked down.