According to Apple, allowing sideloading would put all iPhone users at risk, even if you only want to download apps from the App Store.
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Would allowing sideloading from websites and third-party app stores on iPhone threaten users who only download apps from the App Store?
Yes. By providing additional distribution channels, changing the threat model, and widening the universe of potential attacks, sideloading on iPhone would put all users at risk, even those who make a deliberate effort to protect themselves by only downloading apps through the App Store. Allowing sideloading would spur a flood of new investment into attacks on iPhone, incentivizing malicious actors to develop tools and expertise to attack iPhone device security at an unprecedented scale. Having developed expertise in ever more sophisticated attacks, malicious actors would use it to target third-party stores as well as the App Store, putting all users at greater risk. Additionally, even users who prefer
to only download apps from the App Store could be forced to download an app they need for work or for school from third-party stores if it is not made available on the App Store. Or they could be tricked into downloading apps from third- party app stores masquerading as the App Store." -
Apple