We are talking about the recommended OS on a 2 year old computing device here. Please don't muddy the waters with poor analogies of decade old hardware or terms like "oldest phone". People would be similarly upset if any other company did this. By excusing the company with these poor comparisons, you appear to have vendor bias.
The people want more features and fancy stuff, and that requires more compute power. I don't think Apple should be optimizing for their oldest phones, business wise. (I would not mind personally, using an iPhone 6 myself but have not updated to iOS 11)