Why don't you guys take it in to the Apple Store?
All the devices in my family are on iOS 10.3.3 and with the exception of older devices having worn-out batteries, the latest iOS is completely useable. The oldest of these is an iPhone 5.
The OS isn't the problem here.
Claiming Apple is nerfing phones deliberately for planned obsolescence is kinda funny. When do you suppose they plan for something to be obsolete to force people to get a new one?
A person who's claiming this after their device starts to fail around two years will say it's two. A person who's claiming this after their devices starts to fail around three years will say it's three.