Apple never has to slow your old iPhone down on purpose. Every incentive in the building already points that way. Just look at the numbers. iPhones bring in about $200 billion a year, over half of Apple's revenue. The single biggest threat to that number is you deciding your current phone is fine. And you've been deciding that more often. Americans held phones for under 3 years a decade ago. Now it's pushing 4. Every extra month you wait, multiplied across a billion users, wipes out billions…