For what it is worth, at least judging from the responses to my earlier question in the general discussion forum, this server's present peak population runs at a bit over twice the online player cap for any original (Blizzard) vanilla server (2.5k).
viewtopic.php?f=6&t=2591Given that the (vanilla) game was apparently not designed with any more than that number of players in the game world at a given time, and the lack of adjustments for timers, etc. made on Nostalrius for such a high number as are currently present, I would have to agree with the original poster in advocating for a split realm, or simply a new one to provide an avenue for the excess population (or those who do not want to play with such an abnormally high number of players as is currently present). Beyond raiding and running dungeons, if the server population holds up through time, one would have to think that normal activity in the game world, e.g. farming for materials, questing, etc., along with progressing in the honor system, would be much more troublesome than it was ever designed to be in vanilla, even on the most populated servers.
As far as providing for a lively community goes, while ignoring the details of what a "lively community" actually constitutes, I would say that there is no necessary benefit to that aspect of the game by having a larger number of players playing. Beyond a certain minimum number of players, e.g. enough to actually play the game with some regularity at high levels, as opposed to waiting around for hours in hopes of the last needed member showing up for your group, there is no necessary improvement to the community of the server by having more players. It is quite possible to have plenty of players and a bad community for a game.