This is really the big reason. IOS blocks applications from running dynamic code. The kernel is immutable and replaced on every update, and safeguarding the JS engine is a huge benefit of controlling the OS. Antivirus's really struggle with this on PC because of all the dynamic code being ran that they can't really enforce things like code signing, requiring executable code to be mapped to the disk, etc.
This is going to make everyones phone ALOT less secure. There's a reason android malware and hacks are a big issue and there's almost none on IOS, as the footprint is so low due to things like this.
Apple is by far the most secure, which is why you rarely see kernel exploits and the market for IOS security researchers are in high demand. While the market for android researchers are hot, it's luke warm in comparison. Even in IOS 13, meaningful iphone exploits are hard to come by, while android it's like candy.
Key point, it's going to be difficult to find a way to unlock the bootloader on an Iphone, or a root on modern versions of IOS. Meanwhile you can buy a android phone day 1 and load your own bootloader and get code running in the kernel. This is exactly what a malicious application does. The security boundaries of apps are pretty strong on both OS's, but Apple makes sure apps can't violate that.
This is going to make everyones phone ALOT less secure. There's a reason android malware and hacks are a big issue and there's almost none on IOS, as the footprint is so low due to things like this.