Edit: It would massively increase the usability, allowing any apps to be installed, not just Apple approved apps, making the phone more usable to many people. Back in the day I had to buy a dev account just to load an emulator I wanted to use (it was open source) without rooting.
If they ever do offer sideloading/gatekeeper, it should be turned off by default and turning it on should make the risks crystal clear with a scary alert and passcode prompt. Average folks should be heavily discouraged against using it. Gatekeeper works on the Mac, but iOS devices are both far more numerous and far more personal, so the stakes are much higher.
I know the risks, I want to do it anyway. I don't want a nanny over my head.
For that reason, societies tightly regulate car ownership and driving. More so than phones: I don’t need a license to use a phone, nor do I have to register my ownership or have it regularly inspected.
But technology these days has this same characteristic: Others bear the costs of your decision. Every device connected to the internet is a DDOS vector.
I don’t want you deciding whether to keep your device up to date with the latest security patches, because if you (and a few million others) don’t, GitHub is down for me.
Anyway, if you don't want to deal with how an iPhone works, how about buying a different phone.
You can compile and sign (and, AFAIK, upload to the Google Play Store) an APK on an Android device, using AIDE and perhaps other dev toolkits. No computer needed.
Really, Apple is anti Nothing and pro only one thing: money. Everything else is a corollary. Including ease of use, freedom, and privacy. It's just that those things do matter to me (and GP).
I mean, tha hardware is not open, so developers can't generally implement any proper secure boot schemes (starting from a trusted bootloader), and can't generally control what goes on in the radio modules. Because there are no devices that have those parts open (or I'm unaware of something I want to buy?), there's no security/privacy possible.
Apple can provide privacy there because they're damn huge and they can purchase or design any hardware they want, fully documented to the last every single logic gate. An average free software developer can't.
As for the userland - I believe there are AOSP derivatives + 3rd party apps that result in a reasonably good privacy and security experience.
The only part of their ecosystem that has major restrictions is the App store. Which kind of makes sense as that's one of the few ways in which someone other than Apple could completely trash their platform and make alot of Apple's customers unhappy.
I'm not saying your "anti-freedom" point is completely invalid, but it's really not a major concern for most tech-savvy people, IMO.
https://news.ycombinator.com/item?id=14339031
More timely, the WTC attacks 16 years ago marked another notable event whereafter great freedom was lost in the name of safety.
As that old saying goes, "Those who sacrifice freedom for security deserve neither."
I see your point in the context of e.g. WannaCry and WTC attacks emboldening authoritarians, however this privacy move we're discussing is actually an anti-authoritarian move. It gives the user more privacy and more freedom.
Advocating more lax privacy/security in this case is siding with authoritarian tendencies.