I commented this on a lower voted post that’s about this same idea: free commenter accounts is a terrible idea. Like, that’s how you get spam and sock puppets and harassment campaigns. People who post photos are far more valuable to the community that people who just comment. If anything (you couldn’t actually enforce this) we should charge comment only accounts more and photographers less.
It’s ok if this app is not for everyone. It doesn’t need to have millions of users because there quickly reaches a saturation point where additional users just crowd out current users in anyone’s ability to keep up with their feed.