I think the term is Engineered obsolescence vs. planned obsolescence or taking advantage of a situation to leverage opportunistic obsolescence.
For the majority of the iOS slowdown issues, if the version was not optimized for older devices and Apple took advantage of this to enhance device upgrades ....
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I wish we knew the real reason downgrading is not allowed.
My ex has her 7 on 10.3.3 and her Mini4 on 11.4. More than a few times she wished she could roll her Mini4 back to 10.3.3. In her words “too many issues with 11”. I suspect she is not alone in this.