Technically, they could do that now already. It would just be a single "Meta app" available in all stores, which has in-app purchases for extra features, such as FB, Insta, WhatsApp, Oculus, etc.
They didn't do it in the last 5 years, so there isn't really a reason to think that sidloading is the one thing that is preventing them from doing it now.
This article and the quotes are very carefully worded. The security they are referring too could actually be the security within the app distribution system, which in the current Apple ecosystem is the App store.
Apple can not guarantee the security of the app distribution of a 3rd party system.
However, the security on your device is provided by the operating system and the API an app can use. NOT the the Apple app store.
One can still write and publish an app to the app store, which violates usage terms, exploits a vulnerability in the operating system or hardware, and compromises security. There is no guarantee to protect users against this.
I still want the ability to install apps without involving Apple or the Apple app store. I have working iphones that are unusable because they are "too old" and don't work with the app store anymore. There is nothing wrong with them and I have use cases for them, but Apple has rendered them functionally useless and then says I should buy a new iphone...
An adult tells a child to stay off the table because the child does not have the skills to remain safe on its own.
As an adult, I have actually stood on my table to change light bulbs. I know what the table can handle, how to fall, and the risks involved.
Interestingly, iphones have a setting that allows parents to restrict access for their children.
I do not want Apple dictating how I can use my property, since I am an adult and I know what I am doing. Perhaps adults should limit how children install apps. And administrators can likewise limit how users install apps.
Apple sold me the phone, and it's now my iphone. I want it to be serviceable and usable after Apple ends support. This requires it to function after they block it from accessing the app store. This requires sideloading.