"Maybe your idea of a successful photographer is one who simply "makes money on the side" while you work a "conventional" job. (However, that's not "my" idea)." (Sorry, newbie here and not sure how the quote thingie works).
I had a convention job, now retired after 35 years in it at 57, several years in prior jobs too. I've been taking photos all of my working life, for fun and for pay on the side. I started out taking photos at local car race tracks for fun, then onto selling photos to racers and fans. Got into part time wedding photography for about 25 years, helped pay for my hobbies. Did a fair amount of photography during my full time job because I knew my way around a camera, it was never a part of my official duties. Had lots of great assignments and photo opportunities that way.
Got back into my teen age model building hobby, aspired to have some photos used in the hobby press. Did pretty well there, with over 30 cover photos and now over 850 of my hobby photos published. Made some decent part time income that helped my hobbies and family expenses. My hobby related photos have won major awards (1st, 2nd & 3rd) in many local, regional and national photo contests at conventions and in hobby magazines.
So to me I've been fairly successful in photography, all the while holding down a conventional job that provided me with a full time pension. I never had any interest in doing photography full time, depending upon where my next mortgage payment and other living expenses might come from.
I don't have any idea what a "successful" full time photographer might make annually; I did read somewhere sometime that the annual average salary was under $30K a year, don't know if this is accurate or not. I made a whole lot more than that at my full time job and had the pleasure of doing a lot of photography and making money from it. So was I successful? To me I think I have been.