This will be no worse than what happens on macOS.How will sideloading potentially affect the security of our phones? Does Apple now have to constantly defend against malicious unapproved apps? How much money and effort should they spend to modify software every time a sideloaded app wrecks havoc on iOS?
iPhones don't run MS Windows and don't have those issues so much. macOS, IOS are both basically BSD UNIX under the hood and has its origin in the early 1970s and has always had a reputation for stability and security,
Why? In the old days computers were shared. Many users would log into the same computer and all run their own software at the same time. The OS was designed so that what one user did could not possibly effect the other users. Macs still allow this, I can have several users runing at once