For me, "success" in photography is a rather nebulous term.
For a professional photographer, certainly success could be measured by the amount of income their work produces. If they enter competitions, then certainly the number of awards they receive would define success. I guess also recognition by their peers if they are into such things.
The overwhelming number of photographers out there do not do this professionally, however, rather they do it as a hobby. This is where the term "successful" has many different meanings, and could mean different things to every person. Do they use photography as an artistic outlet? Are they happy with the quality of their photos, regardless of the level of expertise? Do they get enjoyment out of taking photographs? If their photos give them what they are looking for, then yes, they are successful.
Every hobbyist photographer has a different yardstick by which they define "success". For everyone, that yardstick may be a different length and include different scales by which they measure success.