Credentials. If someone states they know something about a certain thing and criticises someone else's efforts then showing their "credentials" you will respect their opinion. If you go for a job interview then you won't be believed unless you show your CV. To be blunt there are a lot of armchair snipers on the internet who like to snipe and they haven't a clue what they are talking about.
The thing with photography is that so much of it is personal opinion. Whether a photograph 'works' is a personal opinion as is a summation of the elements that make it work and the ones that stop it becoming great.
I could if I so chose illustrate my comments with a photo that
does work and whether the photo is mine or someone else's would be irrelevant. My POV would stand or fall depending on the quality of the argument including the illustrating image. Now if I was talking about
how to take certain studio shot then showing my own work would be far more important.
Similarly with the discussion about 'street'. I take a picture and call it street photograph. You call it documentary. The category we place on it is an opinion and says little to nothing about my technical expertise or the qualities of the photograph.
I don't need to be able to paint to be able to recognise some pieces as being impressionist piece of work. Or to be a potter to recognise some pieces as being art nouveau or art deco. I am pretty sure that many respected film critics have never even taken a home video let alone made a movie but that does not stop there being some highly respected film critics out there (yes I now there are far more film critics who are a waste of space but they don't negate my point).
Suppose I cynically post a crap street photo so what does that prove other than I can take crap photos.