Without rendering an opinion as to the effectiveness of getting the flu shot, it’s very likely that your body’s natural immune response to actually getting the flu is the reason you haven’t gotten it again rather than the vaccine.
I used to get the flu every year, though not so severe as to be hospitalized. I haven’t had it since getting the shots. I can’t prove it is due to the shots, of course, but I’ll not run the risk of discontinuing them.