Supervisor insists on writing a manuscript, yet procrastinates on writing for years
In my opinion, if your career is dependent on publication, which is most often true in academic settings, one should just "keep on writing" regardless of conditions. 

Due the time the superviser delayed the writing, if you would have parallely kept on updating the manuscript, two things would have happened.


Supervisor does not write: 
You would have your updated manuscript as a factual proof that you can manage to writ the manuscript by yourself. May be then supervisor may back down and allow you to writ the manuscript. This would be a very helpful exercise for you because writing is arguably the most important and challenging skill in academia. You would get an opportunity to improve your skills further through practice. In future, that would indeed would help in your career. 
Supervisor does write: 
Your parallel updating of manuscript would only accelerate the writing process. The supervisor may probably feel a sense of competition (!) and may do write the manuscript. 


Both cases would have been eventually beneficial for your career. So I would say just "keep on writing".