I tend to agree with the idea that one shouldn't define oneself by a job, especially since one's career is generally dependent on some combination of market forces, institutional gatekeepers, and other factors that are beyond one's control to varying degrees. However, people do tend to put that kind of personal investment into things they spend most of their time on, and especially things they had to spend a long time training for. In order to realize a world where people aren't defined by their career, we would need to first realize a world in which people weren't equal parts encouraged and forced to put most of their time and effort into one, which would require somewhat different economic arrangements than presently exist