The article you cite is a great example of when security flaws are attributed to open systems and not to the weakest link, i.e. the end user. Flaws like these are found within all operating systems, both among mobile and desktop. Regardless of the openness of the applications within. The iOS app store as well as the Google Play Store are both filled with questionable applications. The only difference is that you place your trust in the app store and that the apps within, have been reviewed and verified by the vendor. You have no power over this. The choice to trust these vendors are made for you.
The world is filled with people who are not very tech savvy to begin with, and the smartphone is a huge vector of attack because unlike the PC, it's where users download tons of banking and personal finance apps. You are right in that the weakest link is often still the end user, but knowing this, what's wrong with seeking out a solution in the form of a mobile operating system which does not give its user enough rope to hang themselves with?
My mom doesn't own a smartphone, but if she ever were to, I know I would like to be able to hand her an iPhone, knowing fully well that scammers would not be able to get her to download malware the same way. You try telling my mom "simply don't sideload" and watch her stare back at you with a blank face.
In the article I linked earlier, it was not clear whether the victims were aware of the concept of sideloading or its ramifications, but the modus operandi is often the same. They were tricked into downloading an app from outside of the App Store via an external link or a Facebook ad. That's why I disagree with the "don't sideload if you don't want to" argument, because I doubt those victims would have understood the distinction at the time. It's not as if they purposely went out of their way looking for malware to install. It found them.
You can scold these people for being gullible all you want. The fact remains that there have not been any reports of iPhone users falling prey to this particular sort of scam, not least because it isn't (yet) possible to download an app from outside the App Store, which would presumably fall outside the purview of App Store review.
The iOS App Store is perhaps not immune to malware (though not all malware is created equal, and so far, the best example of a "scam" I can find on iOS are apps which trick the users into signing up for pricey subscriptions, and even then, those can be easily cancelled within the App Store app itself). So the App Store seems to be doing a pretty good job of catching them.
And finally, I like that you brought up the term "trust", because it essentially sums up my entire relationship with Apple. I buy Apple gear because I trust Apple, and I think that is one of the really important aspects of Apple that many people don't understand - that Apple really does go the distance in building up a relationship of trust with its users. That doesn't mean Apple is perfect or beyond reproach, but I would liken it to how people connect with their babysitter or hairdresser. I don't evaluate them based on objective metrics, but instead on how we communicate, whether we trust them to be truthful and fair with us, how they approach a given problem etc, and that's about 90% of what I am buying as part of that service.
You are implying, in the context of this discussion, that choice and security are mutually exclusive. It could not be more further from the truth. This is an unhealthy approach to security and has bitten and will continue to bite more people in the butt.
I hear that a lot, and you know what? The world is not a perfect place, and any solution is only as ideal as the intended audience's ability to grasp it. Maybe the absence of sideloading and trusting Apple to keep the App Store safe for its users is not the best solution as far as ideals go, but I believe it's the most convenient and accessible solution that will result in the greatest amount of good for the greatest number of iPhone users, because the solution is to literally just buy an iPhone.
And to the small number of more tech savvy users who believe that they will never fall for scams and wish to sideload, I can also answer - you don't have to use an iPhone if it's sideloading that you specifically want. Go use Android if you want choice, or pick an iPhone if you value security.