While I have zero problems with side-loading in theory, my biggest concern is that by allowing side-loading you’re essentially creating a back door to bypassing iOS’ security model. In other words, currently, with no side-loading I don’t have to worry about malicious apps of malicious websites, etc (obviously there’s no such thing as 100% security, but you get the point). Without jail breaking my phone (which is easier said than done at this point), I don’t have to worry about the current security measures being circumvented. However, if side-loading was allowed, my biggest concern is that even with it off, it would be far easier for a malicious actor to circumvent iOS’ built-in security since the switch is baked directly into the house OS. I am also concerned this would make it easier for law enforcement to break into a phone.
There are countless examples of malicious Mac applications bypassing macOS’ Gatekeeper settings - even if set to only allow from the Mac App Store. So to say macOS doesn’t have security issues by allowing 3rd party non-App Store apps is simply false.
With that being said, if Apple could develop a way to keep the current security model fully intact and also allow side-loading I’d be all in - perhaps a physical hardware switch? As it stands, right now uses are basically given two choices: greater security/privacy or greater choice. I personally choose security, but fully understand and respect those who choose choice.