I think that's exactly the problem. The real user benefit will be very small, but in order to enable those changes, functionality will be implemented on everyone's phones to support sideloading. In my eyes, this increseas the attack surface against iPhones. Time and time again alt stores have been used to distribute fake apps and malware on Android, and the victims are often those users who haven't asked for sideloading and are unlikely to use it intentionally.
Yes, maybe this will enable an F-droid equivalent on iPhone and it will be great to have direct access to open-source apps. But is this niche addition worth potentially reducing the security of all iPhones? I'm not convinced.
On malware being distributed through alternate stores, yes. For example:
This is just my gut feeling. It is based on not knowing anyone IRL that has willingly installed an Android app from outside the Play Store, but actually knowing people that avoid it because of the potential security implications.
You have to remember that the vast majority of smartphone users are not power users, and not the people who hang out on these forums. While something may look attractive in small circles like these, there are many other factors to consider when targetting the entire userbase.