A pricing model based on number of possible users does not work for specialized resources.
Some databases are of fairly broad utility, but most have interest to a subset of the university.
Encyclopedia Britannica may be considered relevant to all segments of campus, but a nursing, philosophy, zoology, sports or linguistics database is not of broad appeal. Since library funding is a declining segment of the American university budget, any attempt to price a specialized resource on the basis of total headcount is not likely to appeal to collection development librarians. In my opinion, licensing for simultaneous users is the most effective pricing model to date. The library buys what it needs to support the demand, within the limit of available funds. This is the only flat pricing model which relates to actual use of which I am aware.