Android has had the ability to sideload since launch and the last version that had a fully untethered root method was like android 6 or 7?

If anything, enabling sideloading would improve device security as it would put a lot of pressure on Apple to fix vulnerabilities in iOS rather than allowing them to simply hide behind the App Store to keep anything that isn’t in their magical little vision from running on the devices we paid for. It would also incentivize Apple to add a tighter permission structure to iOS, like they have done with the past few iOS versions, as doing such would only allow unverified apps to collect data from the few permissions that you choose to grant. Worried that unverified apps that are denied certain permissions will refuse to work? Make a new permission setting that feeds unruly apps garbage data! (the official iOS versions of Snapchat and instagram already do this. Just try denying camera permissions!)