> iOS is better for privacy but for example if something like fdroid existed on iOS I could compliment the apps on the appstore with free software one that I know also guarantee my privacy.
YOU could do that, but as others have said, the problem is that other people are not going to do that. For the average user, it's a huge attack vector, or let's be honest, it'll be used to pirate apps.
The average user doesn't even know what open source is, and when they Goa head and side load some new game because their "neighbors son really knows computers" and their iPhone gets taken over by adware, who do you think they're going to blame? Do you really think they'll go "oh well, that was the risk I took by side loading." or something? Nope. They'll go "Wow, that was terrible. Apple sucks. The iPhone is insecure garbage!"
That's exactly what would happen and we all know this.
If anything, perhaps Apple could make an "iPhone 12 - Developer Edition" and only allow side loading in that version... and offer no warranty other than hardware. Keep it out of the hands of Average User but the ones that think they're such power users can buy it.