It all really depends on the willingness of the admin to maintain the server alive despite everything, won't you say?
a server with no population could be up forever if the owner wants it that way. MouRO has been up for almost 4 years now, and at least 3 of those years the average population was not above 1, so using your own statement it should have closed a long time ago :v
The moment the admin decides to kill it, it dies, as simple as that.. now you can drop in other "factors" that may influence his decision, such as population, income, annoyance caused by the server, etc, etc... in the end, isn't it but the whim of the admin that determines its fate?