You make the world a better place with the tools you have in this world. If you wait for perfect tools, you will get nowhere.
In the case of AI, your choices are over whether and which models you use. Your choice has zero impact on what others do with it. Let us take OP's case. Their choices are (1) run an AI for some time to make an older phone model usable for some more years, and (2) don't, and have a bunch of people buy newer phones. In this case, option 1 is clearly the better choice, in terms of energy and water use, economics, etc.