All batteries get worse every year of usage. As a battery is charged and drained over and over again it is accumulating cycles and batteries have a limited cycle life. Most are from 300–500 cycles before needing to be replaced. To lower the cycle counts manufactures will often put a larger battery in the phone. A larger battery will allow more time to use the battery before it needs a charge. Like I said before when the battery runs down you then need to charge it back up again and therefore adding to the cycles. So it isn’t iPhone batteries getting worst every year like your question suggest as it happens to all batteries.