It's really not that hard to track player numbers if you really want to - The Japanese do it just fine lol. The way people did it before the game told you was keeping track of search numbers since even if you're in "another world", it will still register you in search from what I seen. So it only really takes someone who really want to know the servers numbers time and effort.
Also, how do you know the servers would never be "that low" if someone else simply can't know the population numbers...?