From what I've read it's actually a battery issue [https://www.vox.com/2017/12/22/16807056/apple-slow-iphone-batteries]. Apple isn't slowing down people's phones to make them buy new ones; they're slowing down people's phones to stop them from crashing due to an aging battery.
Basically, as the phone's battery ages - all batteries age, but lithium ion batteries age in a particularly inconvenient [https://phys.org/news/2015-11-neutrons-aging-lithium-ion-batteries.html] way - it can't keep up with the phone when the phone is running at or near to its maximum processing capacity. The iPhone's battery in particular was causing the whole phone to crash [http://www.zdnet.com/article/iphone-6-6s-sudden-shutdown-weve-almost-fully-cured-issue-with-ios-10-2-1-says-apple/] unexpectedly whenever people ran several apps at once.
Apple released a software patch to fix the crashing issue, but it fixed it largely by imposing artificial limits [https://www.theverge.com/2017/12/20/16800058/apple-iphone-slow-fix-battery-life-capacity] on the processing power of the phone. That is to say, slowing the phone down.
So, not a great move by Apple; they could've been more open about the causes of the problem, and they probably could've avoided building a phone whose power demands would exceed its projected capacity in the first place. From their perspective, they probably decided that slowing down older phones was better for the consumer than either letting the problem go unaddressed or telling consumers to shell out for a new battery.
Ravinoff said:
Not to mention that they make the $50 replacement part not user-serviceable so we can't just buy a battery off Amazon and swap it out in five minutes (like I could with the first cell phone I ever saw, a Motorola Microtac, AKA "a literal fucking brick").
Yeah, this part's especially dumb. Apple has always had a systematic aversion to letting its customers fuck around the insides of their hardware, which leads to decisions like making it stupidly complicated [https://www.youtube.com/watch?v=UgmtNJuqEHI] to remove a battery on your own.
My old Galaxy phone, I can just pop the case off with my thumb and swap out the battery. For an iPhone you need a goddamn specialised screwdriver and you need to take out like a dozen tiny screws. It's daft.