You won't hear any good answer because there isn't one.
You might suggest that I don't have to upgrade iOS. But I have two responses to that. First, is that Apple has begun popping up frequent nag dialogs on the devices telling you to upgrade. Second, upgrades to iOS are the only way to get security fixes. It's one thing to give people a choice to opt out of the newest UI style vs. running older apps. It's something entirely different to require them to forego security fixes. Security issues that by rights should be considered defects in the products with Apple being given a choice of repairing the defects on every platform the defect exists on or buying back the old devices and paying the costs of switching for the people who bought the products in good faith.
It's bullshit that fully functional hardware is completely unsafe to use because Apple chooses not to fixes security bugs in the iOS versions that run on those devices. E.g. first generation iPad is not safe to use online because Apple has abandoned support.
Regardless, these are not exclusive values. We don't have to have one or the other. If Apple ships a device that is defective, it's bullshit for them to say "that piece of hardware is two years old, it's not going to get fixed."
Ask yourself how many if the goods you have are less than two years old? I'm not saying they should bring every feature from all later releases to the older devices, but actual bugs should be fixed for longer than two years.
Imagine if you bought an mp3 & aac player, amassed a collection of mp3 formatted audio, and then the vendor said "oh, we have to do an update because if we don't the player will explode. and we're removing the ability to play mp3 formatted music, but that's ok, you can play aacs." No one wants their audio player to explode, but by the same token it was sold as a device that plays mp3 and aac.
It is very hard to offer support for legacy systems. You should expect more than 2 year support. Software evolves really fast, companies cannot sit and wait. They will lose their hype and money.