The article doesn't clearly says the problem, but there is a problem, and it's none of what you said (although the article does talk a bit about repairability).

The problem, in iOS (but also happens on Android, to a lesser extent), is that older devices are locked out of OS upgrades at a way faster rate than they break down, and in turn, because they run older OS, they are locked out of installing new apps, including older compatible version of those apps.

Simple fact: a 10 years old computer (and probably some 15 or even 20 years old ones) can get the latest Windows/Unix of your choice, but any iOS device that was released more than 2/3 years ago cannot get latest iOS.

Really, it's pretty simple: Apple doesn't care about retrocompatibility. Either you get a new device, or you have to do with what's already installed on it.