No more Apple Pay, for starters. Maybe no iMessage. Things where they can argue they have to turn them off to protect the user, and/or themselves, from rogue apps.
And then you could have a kind of freedom, but it wouldn’t be a very useful one. You can already load whatever you want on your own phone if you have the source and do the build yourself. I can pretty easily imagine Apple making it so you can side load closed apps but almost nobody ever does.
Support is expensive. Apple spends a lot of money to provide what is generally very excellent support. It’s a huge component of the overall brand for many users that stay in their ecosystem.
Apple has gone a long way by sticking to the safest paths, meaning they do things that cause the most customer satisfaction, and don't do anything that can go wrong. That's why Apple abandoned the iCar project because while many would like the concept, a car experience is not something that Apple can have control over; if users get into car accidents, for example, that hurts Apple's brand no matter the cause of the incident. In comparison, making phones, Macs, tablets, etc. is fairly safe as long as Apple retains its full control over the experience (which must be perfect for most consumers).
Because of this, I don't really see Apple allowing app sideloading, even though I personally like it. I think the more pressing issue right now is the 30% cut on AppStore.
The Mac defaults to signed apps. And you need to change a setting to allow unsigned apps.
iOS could easily default to what we have today, but allow the installation of non App Store app after users change a setting and confirm after reading a scary earning pop up, that tells users they may be exposing themselves to data theft, hacking, viruses and money theft.