So, you are suspicious about ‘private collectors.’ That is understandable, as long as you don’t understand. Then again, there is this:

learn from a longtime collector, curator, and consultant. Participants are asked to put together a portfolio of at least five photographs made by others, images they believe reflect their taste, are influential to them, or provoke them. The workshop will focus on influences, how to look at photographs, considerations for analyzing them, and tips for being articulate about them.

It is easy to tell the success level of a commercial photographer: look at their rep, their agent, who gets them work, who gets the commission.
This isn’t the same with fine or decorative arts photographers. Agents can’t sell this kind of work. Agents understand ads and art directors, not galleries and collectors.