In other words, I am correct that iOS is a crippled, ineptly designed operating system, and Apple has an incoherent strategy. That's disappointing.
My Mac laptop sits on my desk next to my iPad. Both are connected to the Internet. I take both of them to coffee shops and airports. They are both computers with operating systems. The primary difference between them is their shape, and how my fingers poke at them, which has nothing to do with security.
If what you say made any sense, it would mean that the Mac laptop has a serious security flaw shared by every other system in existence, besides iOS. Yet, Apple has indicated that the two systems are in the process of merging. Microsoft never had to undertake the complicated, problematic process of merging two systems, because it had the good sense to use Windows for the Surface from the start. Therefore, Apple has an incoherent policy of using certain security restrictions on some devices, but not on other devices and needlessly created two incompatible platforms that it now has to merge.
It's also not clear how the mechanisms you describe mandate that Apple can't allow users to make their own associations. There is some mechanism by which *some* file types are associated with apps. I don't know what that mechanism is, but it's not limited to Apple apps. For instance, my third party voice recorder, downloaded from the App Store, has a file type with an association that works in Files. How it created that association, I don't know. Is it a security risk?
Unless someone gives me a convinving explanation of what's really going on, I have to stick with my conclusion that Apple is technically incompetent. Since I spent a lot of money and invested a lot of time in this crippled, inept platform, I feel obligated to warn other people not to do so. I intend to post a series of videos online explaining this and the many other shortcomings of Apple's current technology line up.
Most of my friends are currently in the process of migrating from iOS and Mac products to Android and PC. Windows 10 whizzed past the MacOS and is almost always referred to with high praise, the Surface is a much better value that iPad, and the current Android OS leaves iOS looking weak. And Apple doesn't seem to have anything else besides iOS and MacOS to offer. No web browser any uses, no search engine, no social network, no ... I suppose I'll be leaving sooner or later too. And I'm sure everyone reading this is seeing exactly the same thing I am, whether you want to admit it or not. In a few years, if Apple doesn't change course IMMEDIATELY, this company is a sinking ship. I predict they'll start closing the Apple stores in less than 3 years.
I briefly entertained the idea that the Apple Silicon might save them, but after taking a closer look, I was very disappointed. I think it's a deliberate deception on Apple's part to claim that the new line of M1 processors is giving performance comparable to the current high end of the Intel/AMD line of CPUs. As far as I can tell, the M1 is a very low power device, but doesn't even come close in real world performance to its competitors, and I think the public has a right to be told the truth.
It's hard to ignore the obvious fact that the Android and PC alternatives are much cheaper, much more capable, and receive much more support from third parties, than Apple. When Steve Jobs ran the company, it used to be that Apple offered enough value that Apple made sense. Now, I really can't say I see any value at all.