My PhD adviser is objecting to my request to change my PhD topic
What you need is a realistic option. According to what you write, your advisor may leave soon and your advisor doesn't want to work with you on a different topic. Consider those as given. You aren't going to "wear the advisor down" or impose your own will on the advisor. That seldom happens and even more seldom successfully. 

If your advisor is satisfied with your current progress and expects completion before he leaves then one viable option is to keep on with what you are doing. 

If you really must abandon the topic, then a different advisor is probably a viable option as well. 

I don't see other possibilities here that have much likelihood of success. Either of those two options has implications on time to completion, of course, as well as on how you act and feel in the short term. But either can be leveraged into a career. Keep your eye on that prize. Not the short term.