One aspect that I didn't think about until I was reading your reply is that as the user base increases, so will the number of people with moderate size wallets (dolphins). These new users will have the effect of spreading around the wealth if they behave independently and up vote subjects of their interest and not just popular content. So it should be true that the $ per user per post should go down, it will actually serve to more equally distribute the available steem. But again this all depends on the behavior of the new and existing steem users.