yes, journals do depend on accepting a certain number of publications... the good news is that for a good journal, that number is far far less than the number of submissions. Journals care most about their brand - that is, if journal x gets a reputation for publishing everything, people will only send them crap, and/or will avoid publishing there entirely for fear of guilt-by-association. What open access journals are selling to authors is rigorous peer review, editing, and help with getting the word out. As more open access journals appear, the competition for high-quality submissions will only increase.