My opinion is that the launching of new server will rarefy the population and ultimately cause unbalance and collapse of the server. It could be an alarmist, but it is never harmful to learn the possibility. It seems the develop team is pretty confident about the prospective population for 3 servers. Noted that the confidence is merely build on previous data that collect for Blizzard to believe that there is actually lots of players fond vanilla WOW (It could be counterfeit). Of course putting the misgivings aside, my personal experience in Nostalrius couldn't lie about the population size. It is tremendous no doubt. Nonetheless population is not the crux of the issue. It is the emerge of the new server that will cause collapse. Just see what happened in Kronos.
Let us see who will play in the new server (for the most part):
New players who never played Nostalrius (unknown)
Players who have not reached 60 in Nostalrius (large in number)
Premium players with almost full gear in current version, and are interesting to build a new character (very few)
Player who reached 60 with average gear, and are interesting to build a new character (large in number, and they could end up on either side depending on server status)
Relying on players' preference, the server population determine which one we stay whether in a profit or non-profit server. Assuming Elysium has sufficient capacity after opening and operate smoothly (The team might just need to reconsider the possibility that the population might not be as many as they thought). Do you see the problem? Most of the original population will transfer into the new server. The sequel comes together with a snowball effect, as more players plunge into new server the old Nostalrius server shrinks little by little. In the end, the overpopulation that occurred months ago will happen again in the new server. At that moment, the distinct divergence in population between Nostalrius and Elysium server will make the team redetermine which server to shut down, since the weekly maintenance effort is not worthy (at least not for a dead server).
Part of the community might say it is okay, since we just want the flourish of the server we are playing on. I am sure that is not the intention of the developer deciding to run three servers in the first place. Either running with Nostalrius PVP/PVE or new server only, the team got to decide. As a vanilla fan myself, I sincerely hope the team ruminate on this. I mean I have a R14 character with Zulian Tiger in Nostalrius PVP, but I don't mind if they decide new server only.