Do you agree with his comments? Are they convincing, or just dispiriting?
You'll hear the word 'pivot' around here quite a lot... very rarely does an idea become a business without being altered by feedback.
I definitely think there needs to be a modification to my business plan, just setting up a stock photography site and "hoping for the best" as it were, clearly isn't going to cut it.
George Lepp was mentioned earlier in the comments. He set up sales directly from his web site and does not have to pay to stock agencies. I bet many photographers would like to do the same - a niche your platform / product could fill. It should be beautiful out of the box, yet customizable since photographers are artist and sure they will want their own look and feel. It should offer easy integration with payment systems (major point of pain), help with discoverability (built-in SEO), embed watermarks for free demos and what not (you should know that way better than me).
In other words don't try to be next Amazon, Zappos, eBay or Yahoo Shopping of stock photos. Be shopify.com of stock photos.
Different model, different niche, maybe even different requirements to the product, but still something to consider. Good luck with your endeavors!