I don't see the validity of this statement, I think it's a cop-out trying to hide the real reason.
Abusing people's desire for security as a false pretend to maximize Apple's profit!
Users who are not sideloading apps on iOS would be just as safe as they are now, how is their safety affected when other users sideload? Exploits of iOS are constantly found anyways, jailbreaks keep happening. Whoever wants to attack non-jailbreakers/sideloaders won't gain much here, at least not in comparison to the gains of the iOS users who want to sideload! If the OS is truly sandboxed well, where is the harm?
These devices (iPhones) are not a connected server cluster, they're individual devices!!!
Quick question:
Why would it be unsafe on phones when it's normal for computers?
Take a guess

iOS basically is a variant of MacOS, they claimed so at least (LOL). Does that mean that they do sub-par security for iOS, maybe get some devs from the MacOS team over to help? LMAO
I can imagine the entire Apple board of directory having dreamt of forbidding software installs on Mac computers just as much as they did on iOS for years now.
In the end, it's just common law. Everybody is used on it on iOS, so most people believe this makes sense and must remain like it is.... think again!
with Phil Schiller's words: "COURAGE!!!"..... c'mon Apple + Apple-Users, how about some courage!
I predict, Apple will lose this argument in court and will have to change their stance on this within the next few years.
____
edit
interesting to see, majority opts for the "courage" option of allowing what has been normal on any computer ever since they were invented.... so Apple, do the same for these pocket computers please!