All things you purchase should continue to run well after warranty expires. They should work perfectly during the warranty period.
Computers should work for at least 10 years with the occasional battery change. This includes mobile phones.
I've still got a 32G 3GS iPhone that works fine except Apple cut it off from getting new apps by not supporting the hardware in newer iOS releases while also not having the SDK for iOS 6.1 with the latest Xcode which only supports iOS back to 8.1.
Apple should support iOS 6.1 in the current version of Xcode because they choose to not support iOS 7 on the 3GS. You either allow the latest iOS to be install on the phone or support the break point for 10 years after the last device is shipped.